JamesShort wrote:
Plus if you blow an engine, James Milko will let you park it nd do an engine swap in his garage

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This is true, with a competent but not Miata knowledgeable 2nd wrench I can pull a motor/trans in 1:15. I'd imagine I could put one back in in a little bit more time. Parts are dirt cheap, the desirable motor is ~1k, the undesirable ones are $2-300 out of Kias.
Budget is also something you should consider when choosing a platform. I want to say I'm somewhere around $9k-$10k deep into my Miata, not counting 'repairs'. I'm likely about 75%-80% of a proper nationals CSP car and about 20% of the money. On some less developed platforms getting a car to where mine is could cost significantly more.
My car, while currently a dedicated AX/Track car, would have a tag and insurance if It was 2 years older. The only thing keeping a plate off it and me street driving it occasionally is OBD-II.
I also agree with Steve, you'll get the itch to at least try a HPDE. If it's a Miata/other convertible that means make sure you can fit under the bar. My height while sitting w/o a helmet on is 33". Around 35" I'd expect you'd start to have difficulty getting a seat combo in a Miata that lets you clear the rollbar and by 36" you'll be making significant amounts of effort to do it. (Cutting the humps off the floorpan & mounting an aluminum seat right to the floor.