I know others will chime in...
I'd first suggest combing through miata.net for basic information. I'm not a huge fan of their forum, but the info on that site is excellent. Miata's are classified in their 3 distict body styles - NA ('89-'97); NB ('99-'05); NC ('06-present). Within the NA category, there's the NA1 ('89-'93) that has a 1.6L engine; and the NA2 ('93-'97). Within the NB category, there's the NB1 ('99-'01) and NB 2('02-'05) various small changes, the front end is the most telling.
I stared my Miata life with a NB1, then bought a NA1 that got a stupid big homebrew turbo; and now I have another NA1 that's more of a driving project car. I've not had much experience with the NC, but I hear it has been a worthy successor to the NA & NB.
Personally, I think that the NB Miata's are the best value for what you get, but you really can't go wroung with any of them. I'd say stay away from the 1.6 cars, due do the possibility of the SNC issue, but if you come across a great deal, I'd still say jump on it.
There's a lot more I could type, but I'm late for a meeting at work, I'll let other's chime in... Definitely check out
www.miata.net for info...