I got the camera today, checked the shutter, and the mechanical stuff. Everything is a-okey.
The battery hatch was stuck, as the guy said.
So I opened it up, and the contacts were all green. I cleaned them up, lifted the bottom contact up a bit since it was pushed down too far.
Fresh batteries in, check for lights...
IT'S ALIVE! :)
So the final amount for a fully functional Pentax MX in very nice condition was - $60.
/Henri