These are cameras up to 60 years old, with unknown histories in most cases. I have a Rolleiflex that looks like crap but I know it was overhauled a year before I bought it, I know who overhauled it, and I know the guy's reputation. I have bought Autocords that look pristine which were useless except for parts. Look, a basic overhaul by Mark Hama, who is the premier Yashica-Mat guy in the US, will be $165 or so. Karl Bryan on an Autocord will be maybe $75 to $150. Either case, you WILL now have no hassles. Short of this, or short of knowing when a camera was last serviced and by whom, you will be taking a chance.
This is just how it is. I've worked on Yashica-mats, Autocords, and Rolleiflexes. Yashicas WILL wear out- the design is solid but the parts are medium quality. Minolta parts are better and the design is better. Rolleiflex parts ooze quality machining and casting, and they STILL wear out.
Seriously, this is what you should do: start following Autocords on Ebay. Look for either well-established camera dealers or people with 25-200 sales of assorted cameras and other things. Check feedback. Ask about the lens specifically, is it clean, no scratches, no fungus. This gives you leverage if they were lying. Be patient. Bid no more than $125, max, and only on a sweet one. Eventually, one day, you will win one. When it arrives, check the lens. If the lens is clean, box it right back up, send it to Karl Bryan. Wait less than a week, and NOW you have what you want: a hassle-free camera straight out of the box.
Again, these cameras are old. Be patient, be thorough, and be realistic.
Yashicas- internal reflection (info applicable to many cameras; I flocked my Autocord, also)-
http://www.flickr.com/groups/yashica_tlr/discuss/72157622734630140/
Door seals being gummed up and needing replacement, you contact Jon Goodman, he sends you parts and instructions for $10 or less, you get dirty for fifteen minutes, problem solved-
http://www.kyphoto.com/classics/sealreplacement.html
There is so much camera-specific information out there that getting answers to all questions about any possible model will become a never-ending thread. You want info/answers about specific cameras, start a new thread with the camera name in the subject line.