Ask if you can go over and see the bikes.
Look for:
-Proper size. Obvious #1 priority.
-Good crankset!!! That's next for me because cranks are annoying to upgrade. Look for one with removable cogs, ideally, and light crank arms.
-Eyelets and braze-ons for fenders, rack and waterbottles.
-Good rims! Aluminum would be good, light and cheap, and has decent stopping power.
-Cantilever brakes! However, I love drop handlebars, so I go for short-pull systems. V-Brakes are also good, but difficult to synch with short-pull levers. Stay away from caliper brakes on any mountain bike.
-Long wheel base and long chain stays. This gives you more room for racks, more stability going down hills, and more comfort for long days.
I'd probably jump all over the Rock Hopper without shifters. He's asking $50, but you might be able to negotiate him down to $40 or $45. I'm cheap... sorry. You can probably find shifters at the co-op, and as I've already recommended, you can get simple shimano friction shifters for almost nothing.