First, you posted at GRM where any question can be answered with "Miata." It's a rule here. What do you want for breakfast? "Miata." What do you need for a track car? "Miata." Etc., etc., etc.. the variation on the theme is "LS Powered Miata" but I am not sure that works for breakfast anymore.
SMs in middling condition tend to go in the $7000 and up range on this side of the country. That usually gets you a decent, workable car that is going to need the typical maintenance items. However, a SM (or SSM as I have) is a fully caged beast, and that limits is usefullness for street use and compromises your safety if you drive it to and from the track without a helmet on.
You said you were looking at the following cars:
"First generation MX-5 Miata, used C5 Corvette (ZO6 posssible but not required), used Factory Five (Cobra replica) spec racer, RX-7, used/older Lotus 7/Caterham replica "
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with the possible exception of the Lotus 7 clone and the Miata, the rest of those cars are big cars that go big speeds and use big part$ to do so. The Z06 is silly fun for the money, but price brakes and tires for that thing - both consumables it will chew through rapidly. If you are considering first gen RX-7s also keep in mind that 12A engine parts, specifically rotor housings, are starting become unobtanium. There are plenty out there, but the newest of those is now 25 years old and the parts tree is starting to dry up.
My suggestion would be to get a decent Miata, add a four point bar to it and hit the track happy. It's a mild combination that is bulletproof, there is plenty of information out there on how to make one fun, and if you chuck it into a tire wall somewhere your total cost may be less than the deductible was for your Viper incident!