The main difference between "color" and "B&W" enlargers is simply that the former include built-in filters. (Some very old "color" enlargers were just regular enlargers that had built-in slots for slide-in filters, but today most "B&W" enlargers can do that.) As Nick says, you can use the yellow and magenta filters to adjust the contrast of multi-contrast papers, so color enlargers actually have an advantage for doing B&W work. (Note this assumes the usual subtractive filtering method. A few enlargers, like some sold by Philips, use an additive color system. This changes some usage details, but not the fundamental color/B&W enlarger differences.)
In some ways a more critical difference is the one between condenser and diffusion enlargers. A condenser enlarger uses optics in the light path to "shape" the light beam for optimal coverage of the negative. A diffusion enlarger lacks the optics; instead, it bounces the light around a bit inside the enlarger so that it properly covers the negative area. As a general rule, condenser enlargers produce sharper prints but exaggerate scratches, dust, and other flaws, whereas diffusion enlargers aren't quite as sharp but better mask negative flaws. Most condenser enlargers have a cylindrical or bulbous head, whereas most diffusion enlargers have a boxier appearance. (This isn't a perfect indicator, though.) Many photographers have strong personal preferences about condenser vs. diffusion systems, but as somebody who's just starting out, you'll simply have to pick one more-or-less blind. Most, but not all, color enlargers are diffusion enlargers.
On a more specific note, if you could post the make and model of the enlarger you have a chance to buy, along with the asking price, somebody might be able to offer some specific advice about it. You could also plug this information into the eBay search engine to see what similar enlargers are selling for. (Click the option to see completed, rather than ongoing, auctions.) Remember, though, that eBay buyers pay shipping. Given the way enlarger prices have plummeted in the last few years, that can be a substantial cost of buying from eBay, but not of a local purchase.
