Hahaha wow, that's not creepy at all (boom07)
Welcome to the club (almost), they're fantastic cars. If it's been sitting for a while like that, there are a few maintenance things I'd do before really romping on it. I'd want to change the oil, transmission and diff fluid for starters. It could probably do with a brake fluid flush as well as new plugs and plug wires. Go easy on it the first few miles to let it warm up properly, and see what you find from there. Hopefully the tires aren't more oblong than round at this point. If it's as nice as you say, many of these things may have been taken care of already, but you're looking at a cost of <$100 for all of that if you do the work yourself.
Most info you'll need for these maintenance items will be here:
http://www.miata.net/garage/
Oh, check to see when the timing belt and water pump were last replaced. Again if you're handy with a wrench you're looking at $200 or so for parts to do this.
You'll want to see if the 91 is an early or late model 91, as the early ones still had the short nose crank that was plagued with failures (mostly stemming from improperly done timing belt replacements). If it hasn't broken yet and the last timing belt change was at 100k when it was supposed to have been, it's probably fine, but def. take a look for anything outlined here:
http://www.miata.net/garage/crankshaft.html
Generally you'll want to look out for a wobbly crank pulley. Honestly, if it's in good shape it's not a deal breaker (wouldn't be for me at least). If it was clear that it's had issues, however, it would be.
If you've got the long nose crank (a maintenance item for the short nose engines), there's a much higher chance of there not being any issue with it.
Hope that helps!
EDIT: If you read this whole thread, chances are you've got all that information already anyway (rofl)