Originally Posted by John E

You will find a wide range of prices, and even the lower side of midrange can be pretty good.

Mandatory:
aluminum rims
quick release hubs
700C size
at least 32 spokes/wheel, in a 3X lacing pattern

Watch the rear axle length. Over the years this has grown from 120mm (5-speed) to 126mm (6-speed standard) to 128mm (7-speed) to 130mm (8-speed). The rear triangles of steel frames can be spread (been there ... done that on several bikes), but know what you are getting into.