1.)My buddy has a '99 aka the NB, Miata's have simple chassis codes NA is the 1st gen, NB is the 2nd, and the NC is the current car. check for rust inside fenders, and under side skirts. Other than typical maintainence his car seems bulletproof. According to him, the NA's 1.6l motor feels a bit more lively than his 1.8l but I don't think it matters too greatly.
2.) He (and just about everyone else competitive) tends to run 15's, less sidewall to flex, and depending on the tires you're looking for, more selection. Are you going to run street tires or race tires? You can find all the big time street tires, Direzza Star Specs, Bridgestone RE11, etc in 15's.
3.) Miata.net, Goodwin Racing, and Corksport (they do every kind of Mazda) are a few I can name off the top of my head.