Hi Christopher, when you go to check out the camera, check out the M/X Sync lever - if it's jammed upwards on the M it means someone's made the mistake of using the self-timer while it was at M instead of X - that's one expensive fix, from what I've read.
Also, take along a roll of 120 backing paper (still attached to the spool) and a take up spool. It will give you a chance to run the camera through it's paces without wasting film and be able to check that the frame counter is working and the film is advancing as it should. My Yashica D is exactly the same as the 635, minus the ability to use the 35mm adapter so, to the best of my knowledge, it works in the same fashion. One thing I forgot about when I first got my camera was that, in order to wind film on, you need to depress the silver button on the winding knob every time. I honestly thought the camera was toast until I remembered that little factoid!
Having said all of this, you're receiving this "advice" from a girl who has had her hands on her very first Yashica TLR and very first medium format camera for LESS than 48 hours! I did do a LOT of reading, though, and the second TLR I got at the same time (a Yashica-Mat) has both problems mentioned above - M/X sync stuck, crank only intermittently winds on film.... but VERY pretty to look at!)
By the way, since I didn't put in an order for 120 film until I knew I had at least one working camera for it, I wound in a strip of 35mm film onto a 120 spool with old backing paper and shot that yesterday - worked like a charm and, unlike the 635's 35mm adapter, this exposes the sprocket holes as well. So, if the 635 doesn't measure up, you can shoot 35mm in any 120 camera doing what I did.
Hope the camera turns out to be in good working order for you. I would pay a hundred dollars for a usable Yashica TLR if I had my chance to get my hands on it and inspect it first without hesitation. With regard to how sturdily they're built, while I have nothing to compare them to, I do find it a very solid build. Apparently, the later 124G have a fair number of plastic components in them, whereas all of the earlier ones are all metal.