I'm sure others here will have better answers to some of your questions, but will provide my 2 cents.
I did about 3~4 years of DE with my 1984 911 targa, bought this car back in 1988, but about 18 years ago and when I was a lot lighter I did race karts for a couple of years.
The car is a dedicated track car, even through I did license it so I do have plates. I bought a totally prepared car, didn't want to race the 911. I narrowed my search to E class cars at that is one of the largest groups at PCA CR, then focused on 944s2 or 968 as I didn't want a Turbo, the NA cars have a little more grunt out of the corners but the turbos are better on the long straights. The car I found, knew the shop that worked on it, and the previous driver showed really good times at a couple of the tracks so I know I have a good car.
I recommend reading the rules (there is a PDF) they are on the PCA site.
PCA E class cars are: 911 SC, 928S, 911 Carrera, 944 Turbo, 944S, 911C4/C2, 944s2, 968, Boxster all with specific year ranges and weights. I mostly see the 944T, 944s2, 968, and 911 Carreras in the class. the 968/944s have better balance than the 911's, it is just easier to drive the 968 over my 911, granted it has power steering, ABS and 50/50 weight.
There are a number of things that can be changed but also what can't be changed, example engine and power train remains stock, I do have RS Barn heads and exhaust. Suspension things have to stay in stock locations but I have a Leda shocks/struts, camber plates, etc. Brakes have to stay stock to what Porsche provided, but you can upgrade to the best brakes Porsche provided for that car, example I have M030 brakes. Wheels can be 1 in wider than stock but diameter seams to be free, I have 8.5x18 and 10x18.You need to keep the dash but you can remove the carpet and some of trim items. Again read the rules.
Hope that helps.
