You are talking about a used enlarger, as the 23C is currently at rev III.(posted 8557 days ago)In the II series, the oldest one I've seen was grey, then moving towards today, green, blue, then black. They went from blue to black for cosmetics only and this is the only change they made at the time. Later, however, they changed the lensboard. The older units have filters that slide behind the lens. While this is a better place for the filter, nobody makes filters for the enlareger that fit the slot. You could mount your own, but it seems like a hassle to me. Or you could make a filter holder for below the lens, which is easy.
The blue and black ones are fine enlargers, the other colors are quite old now, and may be showing their age. The 23C is a fine enlarger. Beseler is still in business, so you can get spare parts if needed. You can go up to 2-1/4 x 3-1/4 (6x9 cm) with this enlarger, thus the model number. Don't worry about color: pick the unit that's in better mechanical condition.
Use a 50 mm lens for 35mm work, and an 90 to 105 mm lens for 6x7. I recommend a 6-element lens from one of the big 3 enlarger lens manufacturers: Schneider, Nikon, or Rodenstock. You can't really go wrong with these lenses. If you're on a tight budget, you might compromise with a 4-element lens, but your best payback in image quality will be with the best lens.
You could use the longer lens for 35mm, but you'll be limited in your enlargement size, and the quality might suffer a litte.