To answer your first question...there isn't a set ranking you need for D1, D2, or D3. In all the Divisions, there are so really good schools and some not so good schools so you really have to look around to see what school fits you best. A girl I know had a USTA national ranking in the 900s and she went on to play Division 1...but the school she played for was a weaker school that didn't win much...obviously if you are looking to play for a strong school like Stanford or UCLA than you would want to be ranked in the top 100s national if not higher. Division 2 and 3 are definitely more lenient in most cases...there are some good schools that are very tough and very competitive, but most of them have a few good players and then it drops off. Granted, there are a few exceptions but again, you have to look into the schools and their tennis teams to get a feel for where you might belong on the lineup.
Coaches can also help you figure out if that school is right for you, so contacting them via e-mail is a good idea to start off. As for them guaranteeing you a spot on the team...in Division 3, technically no, but usually if a coach expresses to admissions that they want you on the team next year, then you will most likely get it. I am not too familiar with D2 except that they DO offer scholarships and D1 obviously does as well. In D1 coaches...if they want you badly enough, you will sign a letter of intent guaranteeing you a spot next year on the team before the regular admission process is even finished for all the other students. If they don't want you bad enough to offer you a scholarship and guarantee you a spot on the team, than you have to apply like everyone else and most likely go on the team as a walk-on...which is tough. If a coach tells you you are in the school, then you are in...I don't know of any coach who has "back stabbed" a recruit.
Good luck in your search!
