What to be afraid of in a Donor Mx5?

[PorkChop]

I wouldn't worry too much about the mechanicals; they are easily rebuildable if you have prior experience and you're on the right side of the Atlantic to get reasonably priced rebuild parts e.g. the crank and conrod bearings would cost you not much over $100 for top spec items. In the UK, you could spend double that for the same items. There are plenty of Miatas running around with 100K+ miles on the clock with no problem.

A small nose crank car (90 - early 91) is less desirable as the crank nose keyway can wear causing pulley wobble. The later 8 slot, big nose cranks don't tend to suffer from this type of wear.

You can buy used engines reasonably cheaply, most people seem to take this route if their current engine is on its way out as it is the cheapest route to take. You still run the risj of having something needing work though.

Could you buy the parts piecemeal if a complete car is 3-4000 bucks? How about getting one from the States?