I can't believe no one has said get an e36 m3. I highly disagree with anyone who said any corvette is the best most affordable car to get. The tires are huge=expensive, good pads aren't cheap, the car has a lot of power which entails using more fuel.
Coming from many track cars I have driven in the past, your best bet is any e36 but preferably an e36 M3, if you have a little more to spend you can even go e46 M3. I like the s2000 as well, never driven one on a track but I have heard all the same devils that other people have stated about them getting bent out of shape really fast. I have also heard once you learn how to tame them they are wicked fast.
I also agree with others, a nicely set up miata would be a lot of fun. I always think making a slower, lower horsepower car go as fast as it possibly can is more fun than trying to control a high horsepower car. Plus all the corvette guys get pretty agitated when you pass them in an old little e36.
Edit: I was doing just a little digging to see what type of D-sports racers were for sale and I came across this. Thought the OP might like to see it.
http://www.racingjunk.com/Sports-Rac...Track-car.html