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I mean, if you want a banger, then just buy any mk1 or mk2 shed with a test in your budget. It'll fail the next one or you pay out for a dodgy test.
If you want a good mk1 or 2, or do up a shitter, I'd budget 2.5-3k. You can get a MK3 for that with fewer problems, probably not as sorted as a fixed up mk 1 or 2, but it's newer, fewer miles, nicer interior, and less susceptible to structural rust.
Thing with the MK2 MX5 is that it's very hard to find one that's mint other than the chassis legs, so what starts as a quick rust repair turns into a mild resto either costing much more time or money than originally intended.
I honestly think Mk3s are starting to get cheap enough that you can get one for the price of a mk1 or 2 that you've then had to fix up.
Or hang on until a MK3.75 enters your price range, that's the cream of the MK3 crop.