Jim,
It's a very good idea. I don't know what used darkroom equipment costs, since I'm afraid to find out how little all my bought new, pre internet equipment is worth. Seriously.

Maybe the approach to take is to offer several equivalences, e.g., 4x5 enlarger - Omega D5 or Saunders LPL XLG 4550 lens Schneider 135mm Componon-S or Rodenstock Ronar. To make it interesting, offer a basic (minimal) set up, and a more upscale version for each set up.

My vote would certainly be for the 4x5 Omega D5 enlarger, plenty of spare parts, solid, adapatable to many third party light sources. The Componon-S is widely available and a very good lens. The Gralab timer for developing and the Besler audible timer for the enlarger,and a Saunders easel would be the basics for a good "value per dollar" darkroom that could be used for many different formats.