Firstly, rust on Miatas isn't always bad but it depends where it is.
If it's on the lower, outer sills below where the door shuts are, then the drain holes are blocked. If there are a few bubbles, then it's repairable - more than that and there's going to be structural issues.
If there is rust on the rear wheelarches, walk away. If there is proper rust anywhere (not a couple of ickle bubbles) then run away very very quickly and do not look back.
Don't worry about the short nose crank thing, that wouldn't affect a car of this age. And there's little difference in the 1.6/1.8 engine for power/speed/etc - some of the later 1.6 versions were knocked down on BHP but at '92/'93, again, this won't be an issue.
The roofs are easy to replace, there will be a million and one places which do this and it shouldn't be too pricey (in the UK a new roof is about £600 fitted). However, if it is visibly damaged, there is a large possibility it will have been letting water in. This may mean the interior will be damp and starting to rot, if this is the case then it will need a whole new interior. If you live in a fairly dry place, then this shouldn't be an issue but there shouldn't be any rust, either.
Over here, a 92/93 one, with average miles on the clock (around 120k) and in half decent nick would be worth about £2000 - £3000, so it looks as if they cost similar money over there.
The one you've seen sounds like a project and a major one at that. The milage worries me a little, if looked after then yes, these cars do really well but the one you've seen has a scruffy exterior so suggests that it's been neglected. I'd expect 250k miles on an average Miata to be at the end of its life.
Have a look at it, ask him about its history. It may be that it's been regularly serviced but not maintained on the exterior. See how it sounds on startup, they do sound rough when poorly and check for the telltale blue smoke out of the back and mayonnaise round the oil filler cap. If anything doesn't feel right, walk away. As for price, $1900 is about right for a roughish one of this age, you may be able to get him down to $1500 if it's in need of huge amounts of restoration. Personally i'd save up and buy a decent one.
Have a look at
www.miata.net - all the information you'll ever need.
Hard top??? *spits*