50mm: get the f1.8, it's sharper, has less flare, and will cost you less anywhere. Neither the f1.4 nor the f1.8 will have outstanding bokeh anyway, so go for sharpness.
This guy here did the comparison:
http://www.pbase.com/miljenko/50mm_vs_50mm
24 v 28: depends. Both are excellent and affordable lenses. The 28mm has the reputation of being very sharp all the way down to the corners. I prefer the 24mm focal length, simply because it has a little more oomph when it comes to take a wideangle picture. Try to compare the angle of view in a store.
Eventually, you might want to have a portrait lens, to take portraits and/or photos with outstanding bokeh. The 105mm f/2.5 is the logical conclusion, but I heard that the 85mm f/2 is equally gorgeous. Either one will give you great results.
I live in Montréal so finding used photo gear here is more of a of a crap shoot than in Toronto, so I bought everything at KEH.com. Check these guys' prices before you buy. You may save a bit by buying online with our strong Canuck dollars.
For your FM2n you
absolutely need AI or AI-s. AI'd will also work. Non-AI will not work.Here are some definitions:
AI: automatic indexing. Believe it or not but it took Nikon forever to arrive with a meter coupling solution that meant easy mounting, open-aperture, TTL metering. It's the basic standard for Nikon lenses, the required minimal.
AI-s: ditto, but has a notch here and maybe a pin there for the purpose of cameras with a Program mode (like the FA or the FG). You can recognize them by the orange color of the smallest aperture on the barrel
AI'd: shortcut to mean an old (pre-1977) lens that was put on the cutting lathe in order to convert the barrel to AI. Has the same functionalities as a normal AI.
Our friend Stephen Gandy has a good writeup on the differences:
http://www.cameraquest.com/aidaiais.htm
And so does Ken Rockwell, with pictures:
http://www.kenrockwell.com/nikon/compatibility-lens.htm