Think about the largest-size film you think you might ever want to try, and make sure you buy an enlarger that takes that size (or the next bigger.) It's very annoying to spend a lot of money for an enlarger that handles up to format size X, then discover you really like using a camera that takes larger format size Y.
Sure, there are a lot of cheap enlargers around now, so you might think you can always pick up a bigger one if you need it... but after enough unsuccessful sellers have given up on eBay and chucked their old enlargers in the landfill, what will you do?
Also, there's no sense buying anything other than a major brand such as Beseler, Omega or Durst. If you eventually need accessories or parts, you'll probably have to buy them used; they'll be easier to find if you own an enlarger that was popular before we all went to Digital Hell.
Personally, all other things being equal, I'd recommend buying an enlarger with a dichroic color head rather than a traditional b&w condenser head. The color head puts out a more diffused light, reducing the Callier effect that makes prints look grainier than they should. And by referring to a chart, you can use the color filters as variable-contrast filters (on variable-contrast papers, of course.)
Finally, the first thing you should do upon buying a used enlarger is align it: set it up so that the baseboard, lens board and negative stage all are parallel to each other. Some people say they do this with a grainy negative and a magnifier; I say don't screw around, get a carpenter's level and line everything up that way. Be sure to check both front-to-back and side-to-side.
What I use: I have a Chromega B66-XL, which is Omega-speak for an Omega B66 enlarger (which handles negatives up to 6x6cm) with a Chromega dichroic color head and an eXtra-Long 54-inch column for making bigger prints. I am ashamed to say that currently it sits disassembled on a lower shelf in my darkroom/workroom, because I very seldom have time to print and need the space in my tiny apartment more than I need to have an enlarger set up all the time. (If I need a digital file, it's quicker to chuck the negative in my film scanner than to make a print and then scan it, and if I need a real print, there's an excellent custom lab two blocks away.)
I really got a lot of satisfaction out of printing, though, so someday -- maybe if I live long enough to retire -- I hope to set up the darkroom again and have some fun. I just hope I can find some papers I like -- all my favorites (e.g. Agfa Portriga-Rapid, Kodak Polymax Fine Art FB Special) are now out of production!
