Here's a start:
DVC is a program that allows you to buy points and then use those points to stay at the DVC villas.
Poly is not one of them so if you really feel you would miss that resort, DVC might not be for you.
You can purchase a DVC contract direct or resale. Depending on which resort you want to be your "home resort" (where you buy your points), there are pros and cons in choosing which option is the best way.
Different resorts have different yearly fees, but they are based on the number of points you own. We own BLT, 180 points, and we pay $3.78/ppt in yearly dues. These do go up each year around 3 - 4%, although Disney has the right to raise them up to 15% (never happened at this point).
DVC works well for those that can plan vacations in advance. You get to book your trip 11 months in advance at your home resort and 7 months in advance at any of the other resorts. We chose BLT because we loved the CR location and only bought because they built there. We wanted assurance that we could get what we wanted so buying this resort allows us to take advantage of that advance booking.
Upfront cost really vary depending on what you purchase. If you want one of the newer resorts they are selling, BLT, GCV, or Aulani, then going direct is probably a good choice as they are limited in resale (and Aulani just opened for direct sales so that isn't even out there yet).
However, if you become interested in any of the other resorts, resale is definitely worth looking into as you can save money this way. Of course, my biggest piece of advice, especially knowing how important the Poly is to you is to take a trip and stay at a DVC resort, whether it be BLT, VWL, or any other one to get an idea how you feel NOT being at the Poly.
As I said, we only purchased when they built a resort that allowed us to walk to/from MK. We looked in to DVC prior to that and even though we could have bought in and saved money over those trips to the CR, we also knew we would not be happy staying at those places so we waited.
Good luck!