By the sounds of your post title, it seems to me you are unsure if you are willing to "take on the burden" so to speak of marrying someone with MS, until you can have some set in stone answers.....I hope I'm wrong.
The fact of the matter is like you mentioned, MS is completely unpredictable, no two "MSer's" are exactly alike. Some go on to live a productive, happy life, some end up in a wheelchair, or using a walker. There is no way to predict the future for anyone with MS. It's a progressive disease, which means it will take it's toll over time.
An example for me personally, was that In September I got a bad headache, I took some tylenol and went to bed. I woke the next morning around 3am to go to the bathroom, when I stood up I immediately fell backwards onto the bed because terrible vertigo had overtaken me. I couldn't look anywhere except at the floor without vomiting, let alone walk. I was bedridden for over a week, after that the left side of my face and head went numb including in my mouth. This all came without warning. After a large dose of steroids I noticed it clearing up after about 6 weeks. Today I feel pretty good despite some minor fatigue. I'm lucky that I recovered 100%, some don't.
If I've learned anything since my diagnosis it's that you shouldn't spend too much time worrying about the future. Live for the moment, and take nothing for granted. Just love her and support her through the journey we call life. Good luck
"You gain strength, courage, and confidence by every experience in which you really stop to look fear in the face. You must do the thing which you think you cannot do." ~ Eleanor Roosevelt