I agree with Georges. The price may be a bit high, but it may just be because of where you're located and the availability of these enlargers there. I got a fine example with all the bits that I needed for $500, but New York is a great market for this stuff. In terms of what to look for:
There are three Durst enlarger head options: (i) CLS501 (dichroic color head, most versatile), (ii) VLS501 (Diffusion head designed for use with variable contrast papers, great if you will ONLY do black and white work), (iii) Femokit B/W (condensor head with opal lamp, lower end model I would assume).
Mixing boxes - If you have the CLS/VLS system you need mixing boxes that diffuse the light and output it correctly for the size of film your printing. FEMOBOX 450 (4x5 film), 69N (6x9 and 6x7 film), 66N (6x6 film), 35N (35mm). You can get away with using larger sizes for smaller formats if need be. I have only the 450 and the 69N and their fine for me for 4x5 down to 35mm. Make sure there are no scratches on these at all.
Condensor combinations - I don't know much about this, because I don't have a condensor head, but I believe that if your model has the condensor head, then you need a FEMOCON 151 condensor for 6x7 to 4x5 film and a FEMOCON 80 for 6x6 to 35mm film.
Negative carrier - The unit should come with a FEMONEG negative carrier. It's a square frame with a hinged top and a couple of slides on the side that move two masking strips inside the unit. As Georges mentioned, it will be accompanied either (i) by two glass carriers (FEMOGLA AN (anti-newton) for the top and FEMOGLA (normal glass) for the bottom, or (ii) by a series of masks that can be dropped into the unit. If it has only the masks, then be sure you have the masks you need for the formats you will develop (the available options are FEMOMASK 35N (24x35mm), 45N (4.5x6cm), 66N (6x6 cm), 67N (6x7 cm), 69N (6x9 cm), 92 (9x12 cm), 450 (10x12.5 cm) and S (special sizes). I have seen many debates on whether negative holders with glass or or without glass are better. I would say with glass is better, even if more difficult to keep clean.
Lens boards - You'll need lens boards for the varying length lenses you may use. I believe the most common are LAPLA42 (for 150mm lens, 4x5 film printing), LAPLA39 (for 80 - 135mm lenses, 9x12 to 4.5x6 film printing), and SETOPLA 2839 (for 35-60 mm lenses, 35mm film printing).
Bits and bobs - My enlarger also came with knobs that hang down from the enlarger head for easy adjustment while working closer to the print. I'm not sure if these are standard, but they're very cool.
Generally, make sure there are no chips in the gearing on the column and check to see that everything moves very smoothly. A good Durst L1200, as the Sopranos would say, "moves like butta" (smooth like butter, for those that have never been to Bayonne, NJ). If it feels jangly, don't get it, it's not right.
Hope this helps. Best of luck.
Leo