Ignoring automatic exposure:
FM2n/FM2/FM: Metering is indicated via LEDs: + 0 -. You adjust shutter speed and/or aperture until the 0 is lit. LED's are theoretically more rugged and easier to see in dim light but sometimes harder to see in bright light.
FE/FE2: You have two needles: one is the metered speed and one is the currently selected speed. You speed and/or aperture until the needles line up. IMHO the needles of the FE series give more useful information. You only have 1 working speed if the battery is dead.
The FM/FE can be used with pre-AI lenses (flip up index tab, stop down metering), the "2" cannot. Top speed is 1/1000 and sync speed 1/125. The "2" cameras have shutter speeds up to theoretical 1/4000 although few will really reach this speed unless recently and fully CLAd. It's typically more like 1/2000th actual. Sync speed is 1/200th (the "n" may say 1/250 but again many only hit 1/200). If you expect to be shooting fast film in bright light, the FE2/FM2 give you about 1.5 stops more range.
Unless you really need the extra shutter speed range or sync speed, the differences are pretty minor. Get whichever you can afford that's in the best operating condition.
P.S. Since you say you already have an MX and a K1000, why switch to Nikon? Foam seals are easy to replace.