$600 isn't too high for a De Vere, if it is in good condition, and has a good set of negative carriers. From your description it sounds as if it comes with two heads - a colour head, and the Varicon B&W head. If so the price is getting reasonable.

On the other hand, I would try and hold to your $500 limit, as that is a decent price for a 504, and it will take some moving. :^)

BTW is it floor-standing, bench, or wall-mounting?