For me it has to be pretty, or at least have the potential to become pretty again.
It has to work. I am not knowledgeable enough for wiring etc. I can do basic cleaning, oiling etc.
It has to have most of it's parts. Unless I know I can get a part at a reasonable price I won't buy it.
I lost a little piece for my Singer 201 while I was cleaning it and a replacement piece is only about 6 bucks so I ordered it. But I've seen things like bobbin cases for Featherweights that people want a fortune for.
I need to stop looking at old machines though b/c I'm addicted to them. I have way more than I need. It's so much fun!