Chance--
No one has mentioned costs. What's your budget for expenses per weekend?
With a lot of these cars, you may be able to afford to buy them, but you might not be able to 1. pay to operate the car (tires, engine rebuild, fuel, brake pads, gear box rebuild, etc) and pay for entry fee, hotel, food, gas, help, etc., or 2. pay to fix the car after you take off a corner.
I can't help you with #2, but for operational costs, check out this
spreadsheet of costs for the various formula and sports racer classes, with a few production cars thrown in for comparison.
Hope this helps,
Greg