Well, first thing I'd say is take your time and look around a bit. If you're in a big city, check Craig's list under musical instruments, lots of folks are practically giving them away, or charing way too much, and finding out that used spinets and uprights don't sell all that well.
Don't fall for the "100-Year-Old-Priceless Antique"-with-pictures sale; old pianos are NOT like old violins; unless they've been meticulously maintained and tuned in a good environment, they don't go up in value much. I recently went out to look at a GORGEOUS Rosewood 1904 6' concert grand selling for $2500, that looked great in the ad's picture. In person, it was a P.O.S., and needed at least $15k of work to make it even remotely usable.
Yamaha, Young Chang, Baldwin and many others make fine upright pianos- good for practice, pubs and dance class use, etc. etc. (Are you sure it's an upright you want? Is space an issue?) Good uprights are actually small baby-grand piano cases turned vertically, unlike spinets which are just cheesy and never sound good, regardless of what you do to them. (It's partially due to the thick - instead of long - low strings in spinets. You can't cheat physics in this case, and the ear knows it!)
Beware the "project" piano - one that looks good, but needs strings....and then maybe sounboard work....and then hammers, action, etc. You can pay $2500 for a nice looking piano in bad shape, only to have to spend another $5-25K getting it up to spec. Make sure you bring a competant tuner with you to check it out - not unlike having a mechanic check any "used car" you might buy.
That's a start, anyway.....