The first thing I absolutely do before I bring the machine into the house is check for critters, if its really dirty I do a surface clean in the garage. Have not found a mouse nest in the bottom of a case yet and I hope I never do!
I then bring it in and really do a good surface clean with machine oil. All of the machines I have bought have been in remarkably good shape , no rust on any of them.
Then I download a manual and look the whole machine over. I look for missing or broken parts, I check the electrical wiring end to end with a flashlight. If its good I plug it in, if not its going to my OSMG, I'm a newbie, not going to do electrical, YET, lol.
Then I oil as directed in manual. Do cleaning of feed dogs etc. Try to diagnose any problems and fix them so I can get a good stitch.
I really need a "can this machine be saved" type if machine to pull apart , clean and put back together system by system.
in fact I'm looking forward to that machine.