Best Track Car

[Three Peaks]

You can get into a FF5 Cobra spec racer used for around $20-25K and have a bunch of fun for little money. Brakes last most of the season, the engines aren't run hard as they are basically stock, and all the other parts are pretty basic and cheap/easy to replace. Even tires would last a good while. Probably most fun for the money.
Each of the other cars you've asked about is going to be a very big step up in cost and complexity, especially the GT40, which I doubt you'll be able to campaign in any race series for less than $125K initial investment and a very good pit crew and expensive spare parts. Transaxles aren't as easy to source as a 5 speed transmission and GT40 parts aren't exactly off the shelf parts like Cobra and Coupe parts mostly are.

David is absolutely right about the Miata racers. Club racing is the best way, IMO to get into racing and learn how to drive well. Then you can decide if you want to move up to high HP or just continue to have fun. Ask any Cobra driver who has done a track day and gotten passed repeatedly by the Miatas in the field...

Another suggestion if you want speed at moderate cost. I've been watching the Mazda open wheel racers for several years at the SCCA races around here and they are having more fun than anyone on the track. Running sealed Mazda 4 cylinder engines in an open wheel and very light car is a recipe for fun and very tight competition within the class. And if you check their lap times, most Cobras except for possibly Morris's super prepared and expensive Cobra will come close to competing with them. And the best part, you can get into a used one in good condition with many spare parts for around $15-20K to start. Same as the Spec racer Cobras, the parts are off the shelf Mazda parts mostly, calipers, rotors, pads, wheels, engine parts, etc... and are cheap and easy to find and change. This is all information I gleaned in a couple quick afternoons BS'ing with these guys at a couple races.

If it were me, I would go to a couple races and get pit passes. Talk to the guys who are having the most fun and start asking questions. Go from there.

Bob