Hello and welcome, plenty of info here if you search but briefly in my opinion;
(dates and pics are in the histroy section)

There are no UK full power spec NSR's, they are all Jap imports.

NSR250 mc16 - 45bhp and hard to get past that, some parts very hard\impossible to get now.
NSR250 mc18 - 45 bhp, easily made up to 60bhp, some would argue the best of the lot, certainly the best engine for power, very similar to gp bike of the same era. Specific mc18 parts getting hard to find.
NSR250 mc21 - 45 bhp, quite easily made to to 60bhp, modern looking frame and fancy gull arm mean it handles as well as anything, engine taken straight from the mc18 and made worse. Parts in good supply.
NSR250 mc28 - 40bhp, a bitch to get made up to 60bhp, mc21 frame, single sided proarm, more electric toys than it needs. Just about all parts still available.

Reliability and maintainance wise as they are all basicly the same engine they're very similar,
Prices, they just get more and more expensive, mc28's are not cheap, watfordhorn only got £550 on e-bay for a tidy mc18, I got offered £2500 for my mc28 the other week and I doubt there's a second a lap in it between them.

Realisticly an mc21 is the best buy, but then again, realisticly a modern 4 stroke is even better.
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread