MX's are indeed not rare, but flawless working MX's are IMHO. I've repaired quite a few and these are the weak points to beware at:
1. the shutter is not electronic and made of fabric in stead of thin metal alloy; therefore more prone to wear (comparable to the old Barnack Leica's)...I've seen a few were the shuttercloth was or came lose from the shafts;
2. the light meter diodes are very cute presented in what may look at a round translucent dial; however that dial is operated by a set of cooperating wheels connected with a very thin metal wire. Over time the wire becomes lose: most of the MX's I've had in my hand, had slippery wires resulting in a bad / wrong readout of the diodes.
3. as the old MX's almost all old Pentax camera's are in need of new light baffle material.
All these 'repairs' can be done by yourself, but if the camera is in need of those, it should only cost you a 'bargain' price....