I have been belly dancing for years. I started out taking a beginner class and supplementing it with dvds, and then went on to join a dance company and traveling-taking seminars many of the world's top dancers. From there, I ended up teaching my own classes, and now I direct my own dance troupe, and perform on a regular basis.
As far as dvds are concerned-if you are ONLY doing it for exercise, then doing whatever dvd you want is just fine, as long as you are moving. On the other hand, if you actually care about proper instruction and want to learn it correctly, I recommend HIGHLY taking an actual class. If you don't have mirrors in your house behind your tv, it is hard to see if you are actually doing the movements correctly, and an instructor will make sure that you are using proper form. There are also many other things that an instructor can teach you about belly dancing that a dvd cannot.
As far as dvds, there are SOOO many out there, if you don't know what you are looking for, it is hard to choose. Here are some that I recommend for beginners, especially those doing it at home:
~Bellydance Fitness For Beginners (Veena & Neena) Basic Moves
~Discover Bellydance (Veena & Neena) Basic Dance
~Perfect Hips-Michelle Joyce
~Body By Bellydance-Michelle Joyce
~Amira's Belly Dance 101
Now, I recommended two of Michelle Joyce's dvds...but I wanted to note that some of her dvds are not meant for the general public, they are meant for professionals. So, I do not recommend that you purchase Pop, Lock, and Shimmy...or Drills!Drills!Drills!, or Secrets Of The Stage series, because if you are doing this only at home, you are going to be EXREMELY lost when she jumps into advanced moves playing finger cymbals.
Another series that I HIGHLY recommend is the Private Lesson series by Ruby Jazayre. You can order it directly from her website. It is a 3 part series that has a posturing section, upper body section, and lower body section. Ruby is a phenominal performer, and a wonderful teacher.
As far as the body, and what it can do for your body, basically, I want to know what you are trying to achieve.
Belly dancing is going to be like anything else-a Pilates system, a treadmill, or whatever. If you USE it/DO it, you are going to get results, if you don't...then you won't. You will get out of it what you put into it.
Belly dancing is a wonderful exercise form, and it provides both cardio benefits, and toning/strength benefits. I also do strength training, yoga, and walking...and I have found that belly dancing does things for me that other exercises do not. For one, I have noticed that when I do strength training exercises for my back, that I get my upper back a lot. When I do yoga or Pilates, I get my lower back a lot. When I belly dance, I feel it more in that area inbetween...right around where your bra band sits.
I also feel that I get a much more well rounded abdominal workout that crunches can give me...my upper and lower abs are worked, and my obliques are HIGHLY worked.
My thighs and calves are very strong...as are my shoulders. I also have more grace and musical rhythm. My butt and hips are definitely still round...but I could crack a walnut with my cheeks.
Also, your diet is going to play a part as well. There are belly dancers of all shapes and sizes. If you only dance 2 times a week, and don't watch your diet, then don't expect too much weight loss/transformation. However, if you are eating to lose weight, and you exercise 5-6 days a week, you are going to get much better results.