If you have eyelets at the dropouts then you have to use them for a 50lbs load, carrying that load with a seatpost rack is asking for trouble.

If you have a steel frame you can have eyelets brazed onto the seatstays. Otherwise one of the better quality P clips may be OK.
Any standard rack will fit.
You need 3 legs, preferable with all-round triangulation. Dog-leg stays are useful for flexible panniers but not needed for good ones.
Do you need a top-plate? If you already use fenders then an open frame top is more versatile for attatching stuff.
Make sure the rack is compatible with your pannier clips. Most good racks fit standard panniers. Some of the cheaper ones use poor design such as doubled-up section of tubing which cant accept locking clips.
A rear lamp bracket is always useful.