Well there are 3 generations, the first gen being the cheapest and simplest. They are definitely the most raw and fun to drive, but also the most rough. With each generation you get a bit more comfort, power, and curb weight. But even so, the heaviest Miata will be not much more than 2400 lbs, and the lightest about 2100 lbs. If you are a big fella I'd go and test drive one, they might take a bit of getting used to. The first gen and third gen are the roomiest.
Here is something to look for:
http://www.miata.net/garage/crankshaft.html
Other than that, pretty much do what you'd do with any other used car. They are tough little suckers.