Older pans tend to be thinner and smoother. That's nice--they'll season faster and weigh very slightly less. I'd check for a flat bottom, partly because I have induction, but also just as a sign of decent craftsmanship. (Obviously that's not what you'd look for in round-bottomed or footed items.) A smooth interior that isn't rusty is best, but even pebbly is OK--just a little harder to season.

As far as using a used pan, I'm right there with you. I probably have an old post here about should I use this pot and how do I strip it since for all I know it was used as a camp toilet. Whether you burn off the old seasoning or EasyOff it, it'll be clean enough you won't have to worry about meth or poop.

Look for a nice fitting lid, too. Some brands are more famous than others, but I think there are plenty of the less common brands that are just as good.