I have an FM3A and it's a great no-nonsense camera. It has centre-weighted metering, manual (mechanical shutter) or aperature priority (electronic shutter) setting, and has only one small battery that never seems to wear out. The disadvantage as a wedding camera might be the lack of autofocus, but never having done a wedding........(when I say "done" I mean photograph of course - I have been a participant in a couple).
One big advantage as far as I'm concerned is that it takes a lot of good (AI and AIS) lenses that can be found at a reasonable price on ebay etc. I have a 28, 50, 105, 75-150 and 43-86 zooms. The 75-150 was a fashion photographer's favourite apparently.
It also supports TTL flash. You can buy a motor drive for it, but presumably that is rather noisy. Hand cranking and focussing might take a little while to get used to at a wedding though.
Let me know if you have any other questions I can help you with.