I could have written your post, word for word, except my ex SD (note the word ex) was 20 when I met her Dad 

She did ALL the things you describe...and my two adult sons noticed and commented on her rudeness and disrespect.

Quite honestly, I don't think this will get better....the daughter won't change ...he won't have the balls to stand up and parent her. 

Sorry but that's my lived experience. You decide what's best for you.

" I guess my question is should I be discussing this with him now so he can possibly start rule changes now?"

If he was going to start changing the rules, he'd have done it by now. Not going to happen. I also suspect that he'd rather stay in the comfortable place that he already has with his mother rather than find a place with a separate unit for her and making her feel unwanted. My money is on him telling you that he'll let you decorate the place he already has so that you feel more at home... I doubt  he'll understand why you want a new "your" place. 

Either way, make the things that really count for you clear to him NOW. Don't waste any more time. It's only fair that he hears exactly how you feel. He doesn't have to make any or all of the changes you want, but he does need to hear what's important to you.

Good for you for not being blinded by love and seeing all the red flags. You should let him know your concerns and see if he makes changes (on his own, without pressure) as a result, but it's unlikely. He's the type to be too afraid to set limits on his daughter for fear she won't come over anymore, and that fear is really hard for these men to overcome. I'd plan on dating and living apart long-term, and in the meanwhile, set your own limits on not making plans based on his kid's whims. 
 

He may be a good guy but he's a lousy parent, and chances are good that he can't stand up to his mother, either. 

I guess my question is should I be discussing this with him now so he can possibly start rule changes now?

There are not going to be any changes, so what you see, is what you get. And, if you think it ends when the kid turns 18 or moves out on their own, guess again.

The kind of parent he is, is the kind of parent that he is, and that will ALWAYS affect your relationship, whether you live together or not.

Yeah, good luck with the mother situation, as well.

The problem is not always the amount of baggage, it's how the owner handles it.

Potential nightmare? Yep.

I think the biggest thing to address is the schedule. Your SO needs to have that settled so you both aren't twiddling your thumbs waiting for SD to give him the time of day. 

As for the other rude behaviors, I kind of wish my SD would hide out in her room. Be careful what you wish for there! Also remind yourself that you can't care more than the bioparents. Focus on your kids and being the best mom to them you can be. Set a good example but I wouldn't push your SO to change how he parents SD, he probably won't change and it will just create strife. 

What is at all appealing about a relationship with this train wreck?

HELL YES!

I couldve wrote this, minus my own bios and my DH's mother living with him. 

I was not as bright as you, kind lady, I married the shit guy. We divorced.

Like all have said above, he will not change. He is scared of his rude DD and probably is a mumsey boy? IDK on the latter only you can decide.

Do not give up your good life, the life where the spoiled rude brat will control all aspects if you move in. Not to mention his elderly mom who may have you washing her clothes, cooking her meals. Ya know the servant. Again my apologies if his mom is a good one...I am seeing too many red flags. Not to mention your poor bios having to deal with a shitty step sister.

Blessings

This is the perfect time to communicate with your SO and set your expectations.

"SO, we've discussed living together after my youngest goes to college. However, for me to feel comfortable with that, these are my expectations for living together. What are your expectations? Can we compromise on X?"

Having that conversation now and seeing what he says, if he is putting things into place now, etc will give you a lot more data to make a decision in 3 years.

However, don't forget to address things that bother you now. If you're traveling to him on weekends because he is supposed to have his daughter, but she ignores him and he doesn't show up to collect her anyway, then you need to stop going to his place. Tell him that, when he collects SD, you'll be there. However, if he isn't going to get her, he can show up st your place. He'll either figure it out to see you or he'll make an excuse as to why it's "not possible" (which is BS). Stop spending your money to board your animal and wasting time going only to his place if you don't have to.

Lastly, what kind of relationship do you want in the future? Are you okay with this kind of arrangement, or are you looking to live together and be married, or what? If the current arrangement isn't what you want, then you need to consider how long you're willing to wait in order to get what it is you want.

Good guys are a dime a dozen. Most people are dateable. But all because someone is dateable and "good" doesn't mean they are right for you or available for a long-term commitment. With how you've described this man, I don't see anything wrong with dating him, but I certainly wouldn't be looking at him and this situation as something serious and long-term. It's perfectly acceptable to have "good enough" and "seasonal" relationships. They don't have to all be long-term to be good or end in chaos to know it was wrong for you. Being casual and having fun - which includes not giving up all your free weekends to board your dog and stay at his place to appease his daughter - is always acceptable.

Nightmare? Yes. Like a nightmare onion, with layers of "Hell no!"

If you're already getting bad vibes in a LDR, you can assume its even worse when you're not there. Listen to your gut and find someone who either has less baggage or is better at handling the baggage they do have.

The daughter alone should make you want to run far and fast.

Having been in StepHell for 14+ years, your story leads me to believe nothing will change for you. Or if it does, it will only get worse. Consider how you feel right now, living an hour apart and in your own home. Now imagine being impacted 24/7. You and your kids.

As several have said, time to communicate this now. I'd have a private meeting (dinner or even just meet for coffee where it's just you two) and bring up ALL these points you mention in your post. At least give him the opportunity to know what the issues are and give him a chance to begin changes. Or, if you are fairly certain nothing will change, then I'd just move on.

Life is short. While DH and I are doing much better these days, it's been a tough journey, one I'll never repeat. Should something happen to my DH, I'm done with marriage and co-habitating. 

I know fathers having their kid every weekend is common in the US but it's a completely foreign schedule to me - I live in Australia and have never heard of anyone with that schedule here. I would never have moved in with DH if we had the skids every weekend, couples need downtime (without skids) together to build that solid foundation. 

Otherwise, I think everyone else has said it. Make your expectations clear and you will quickly see whether DH is willing to respect your needs and compromise. I think it's highly unlikely personally and I hope you can move on and find someone better. 

I agree with the others. Ive heard of those who are "together living apart", but this sounds really difficult to overcome. If his daughter is bad now, just think how shell be as a teenager. When you move in. When you (maybe) get married. 

Meaning if shes rude now, all heck will break loose if there are any stressors like major changes.

Id stay apart for now but discuss the things others have commented on.

ive been in this situtation before and i would honestly save yourself the trouble and dont live together because lets be honest,, you will end up having to move out anyway as the situation only gets worse.   once you move in , the jealously with the SD will start, she may ignore you now but when you live together , these type of step kids start to use you as target practice.  She will start to do things on purpose to atempt to get you and DH fighting and drive you away,,and it usually works as these type of parent will ALWAYS take the skids word for it over yours.  It happened to me,,ignored all the red flags from DH and his parenting and ignored all the red flags from the kid thinking "oh hes just a kid".   I put up with it for 6 years in my own home until I couldnt stand being so irrelivant in my own home and life.  The kid ruled the house and theres nothing i could do about it ,, and dont think your DH will be in your corner ,,,nope ,,,,you just become the girlfriend who doesnt like anything about his kid no matter how hard you try!  Continue dating by all means but be very very hesitant about moving into this situation .  good luck 

My advice is, stay in control of your own life until you see the change that will make things work with the relationship.

I've made this mistake myself far too many times. Giving up my independence to move in with a SO.

While things are difficult it's best to keep your own place as a safe havern. 

Oh I wish I had done this myself.  It would have eliminated so much heartache.  

... seriously date or make any sort of commitment to anyone with children under 18. Even after the kids age out of any custody orders or real (as opposed to emotional/imaginary) demands on the parent's resources, look long and think hard. Keep your wits about you. Can't find a man without baggage? Still a hard pass. No man is worth the grief.