Interesting question. For me, I have three criteria - condition, price and querkiness. If it has ANY rust, I just keep walking. There are too many machines that have been well-taken care of and need just a little work to get going. Price is a big one for me. I don't like spending more than $100 for a vintage machine. Mostly because it will cost me another $100 for the tune-up once I buy it. There are exceptions of course, like a Featherweight. If it is querky, I might get it. I bought a toy sewing machine and I would consider getting a second one if it was from Germany and well painted.
I have bought sewing machines I don't need just because of price. I picked up a 301 for about $50 at auction. The description said the needle wouldn't move up and down. Turns out it was because the knob at the end of the flywheel was disingaged for winding a bobbin. So if it is cheap, just make sure it works.