I got a well beaten 820 for $20 on CL.
Someone had tried to "fix" the shifters, so I scabbed on a cheap pair of friction shifters from a Wal Mart bike and I had a serviceable back up/parts donor.
Someone then stole the rear wheel from my 86 Rockhopper, so the TREK gave up its wheelset (Free Hub) and now has wheels off a RoadMaster. It's still a serviceable back up!
I was also able to try a different length crank for just my time swapping them. Finding out how much better a slightly shorter crank worked for me was well worth the $20!

Also, it's a good "practice bike" for attempting a repair that is new to me, as a confidence builder. Even if I screw it up, I've still got some "spare parts".

Don't get crazy with upgrade$ and it'll serve you fine as a 2nd bike. Only do an upgrade if you find a part too cheap to ignore!