WElcome to the DIS!! This is a great place isn't it??
There are many threads and forums that have everything you want to know on them, but the searching can get somewhat daunting. I will answer your questions best I can.
First of all there are obvious big differences between buying direct from Disney vs. resale. Irregardless how you purchase, you will have the same benefits as any other member. Disney is offering only Saratoga Springs right now (other sold out resorts may be bought through Disney on a waitlist).
You will get points immediately when buying from Disney. Also resale when contract has closed and all documents have been processed with Disney (and if the points were available to begin with).
We have gone back and forth on the matter many times, and decided on a small resale contract (unfortunately Disney ROFR'd it). I personally just didn't want Saratoga Springs as my home resort..just my personal preference. We are now HOPING with CROSSED FINGERS that the Contemporary Resort Villas will be the next
DVC property. If this is the case we will finance through Disney
You can always do add-ons whether you buy from Disney or resale but again it is a waitlist only basis and you never know how long it could take.
Everyone has an opinion on buying through Disney or resale, it boils down to a personal preference.
I think that I am very knowledgable about the DVC but there are MANY more on this site with MUCH more info. than myself. Take the time to go through the many threads here and you can figure out which is best for you.
Good luck!!