Depends on how handy you are, and what you're going to do with it. Are you adding to your own collection, or do you just like to tinker and learn? The requirements for either are different.
If I'm adding to my collection, I look for unusual-something I don't have one of. I seem to have a kink for friction drives, and now have half a dozen or so of them. Is it in good condition? If it's not, can you fix it, or are you willing to pay someone else too? Do you have a budget?
If I'm looking to tinker and learn, I care far less about looks-I've bought some real dogs on the theory that they're already beat, so nothing I can do to them is going to make them worse. Ugly ones are usually cheaper, too.
If you're not keeping it, what will you do with it? If you catch and release, do you have a steady supply of people to release to? Most of them are likely to want a basic zigzag and a 'real' bobbin, not a vibrating shuttle machine, which influences what you want to buy. It's an expensive hobby to buy, fix and give away, but I know someone who regularly sends machines to either Afghanistan or Africa.