I would rent directly from the owner, rather than a management company. When you rent directly form an owner, you know exactly what you are getting. If it is a mgmt. co., then you get whatever they have available the day you show up, and it may not be the "model" shown on their website.
I agree that you should try to get close as possible to WDW if that is where you plan to spend your time. Emerald Island, Windsor Palms, Formosa Gardens are the three nicest and closest developments, IMHO. I used orlandovillas.com which has been great. YOu plug in your dates and needs and then you start getitng replies form owners who have villas that meet your needs, directing you to their websites. Most of the owners are from the U.K. The villas are "verified" by the webmaster, whichmenas he has checked to make sure they are owned by the people who claim to own them. On the DIS, I would try the Orlando forum for more info.
Orlandovillas.com also has discussion boards where you can ask questions. Other websites to try are vrbo.com and villas2000.com.