If you have a choice, get a color head for color work or variable contrast b&w (which is what you will start with, and might never leave -- variable contrast paper b&w is the simplest thing to print), and the biggest enlarger that will fit in your space.
Bigger prints require, generally, a taller machine. If you only want to go 11x14, pretty much anything will do. Bigger gives you more flexibility, though.
The biggest single thing to research on any candidate enlarger though, is whether it has alignment adjustments. In order to pull good prints, the negative, the lens, and the easel MUST ALL BE PARALLEL to a fairly high precision. Cheaper enlargers do not allow you to adjust the alignments of these (you only need to be able to adjust two of the three, of course). Older/used enlargers will generally be out of alignment. If they don't have a system built in to adjust it, you'll have to improvise something (which can be done, but sucks).
The next thing to look for is whether negative carriers are included, and if so, which ones. These can get surprisingly expensive, although you can fabricate your own.
Then get good lenses. You can pick up very decent enlarging lenses on
eBay for next to nothing.
Brand of enlarger doesn't really matter. Really outlandish ones might require negative carriers or light bulbs that are impossible to obtain, however, so if you have a candidate that's not Beseler or Omega, do a little web research on it. Actually, always do a little web research on it -- for all I know, Beseler built some weirdos that you can't get parts for too, and you wouldn't want one of those either.
So: Alignments, neg carriers, and bulbs. Everything else, you can work around pretty easily, and a color head sure is nice.
Me, I'd choose the enlarger that came with the timer and the trays and the tongs and the safelight over the one that comes with nothing. If it comes with a bunch of free paper and chemistry be realistic: This is a way of saying that it comes with a bunch of free garbage.