So a 911 is the one to get for performance driving(auto cross)? What is a 968-944-951 good for?

Which 911 to look at? What year did they switch to liquid cooling? What about the all wheel drive porsches are they trouble?

Which 911 is the most simple with less headaches? Air cooled or liquid cooled?

I don't want some limited edition collector. I want a porsche that I can drive and have fun with and which performance mods work the best on which engine?

Well if you want to modify the car, then get a turbo as you can make the cars very quick whether it be a 951 or a 911. Both are good for road courses. Just remember, if you are going to buy a 15-20 year old car, you will have a lot of maintenance $$$ to deal with, especially once you start throwing hp at it. If your budget is 25k, then don't buy a car over 15k, because you will sink 10k into a used turbo car in a heartbeat, whether it be a 951, 911, etc. Especially to get the numbers you are looking to get.

911's went liquid cooled in 98/99 with the 996.

AWD depends on the model, but are fairly dependable.

the older 911 is a better performance car. the newer cars are a one and done deal. there is no porsche that is simple with less headaches. if you are not mechanically inclined and on a budget,you dont need a porsche.especcialy if you are into drag racing they are not made to thrash on they will make it very expensive ffor you.