I still have my FM2. Great camera. Robust, tough and fully manual. The battery only powers the meter, so you can continue to shoot even if you lose battery power.
I last used mine several years ago; however, I've heard from others that - since some of these are getting up in age - you may need to do a CLA. I've heard that some of the material in the mirror box can start to degrade and needs to be replaced. Haven't done it myself, but have been thinking about it.
Check with a local reputable repair shop for the cost to refurb; may impact your economics.
I think the problem has been solved since the OP here forgot the sale post open (but he now declared he doesn't want to sell it any longer) but I still have just a question: what's in your acronym a "CLA"? Some kind of check?
Althought I've understood the rest, this is a word that I don't know.