I can usually find good wheels & wheelsets at the local bike co-op or on Craigslist for about $50 per wheel, or less. Often requiring some grease and adjustment. Spin by hand to verify a wobble, and carefully inspect for cracks near nipples.

I prefer narrow rims (19mm or 20mm outside width), although,perhaps that is more traditional, with a lot of people going to 23mm outside width.

I can't think of anything to really avoid. Do you have/need disc brakes? The disc hubs don't hurt much, but there are a few disc specific rims that could be problem with rim brakes.

Tubeless Ready rims are often harder to mount tires on.

I have been avoiding the new Mavics, and anything that appears too proprietary.

Be aware of differences between clinchers (and tubeless) VS Sewups/Tubulars.