I road Marins for years, if you get a hard tail, they should be in the $600 range and fit your needs. TAB is right though. Hit Craigslist and buy used. I'd recommend one with Grip shifts, and if it doesn't have them buy some toe clips. They're about $12. They make it much more efficient as you're applying force on the upswing as well as the down swing, you just need to remember to jerk your feet out backwards rather than sideways or you'll face plant.

Two other things.
First, Go to a bike shop (not a sporting goods store but a dedicated bike shop) and look at new ones so they can tell you what size frame you want, heck, they might have some good used ones as trade ins or on consignment).
Second: Once you buy it, if its used, take it back to that shop for a tune up. This will set you back about $70, but they will lube the gears, cables, tighten the chain and align the breaks, You should do it every year or two anyway, just like a car. If you're going to ride it off road, buy tire liners that will protect the inner tubes and get them to put some "Goop" or something similar into the tubes. Also spring for a small frame mounted pump, a little bas that fits under the saddle with a new tube, the tools required to install it. (all this is cheap, maybe $50-$70 total, because I speak from experience that walking 15 miles back to the truck with a bike over your shoulder sucks ass). Also spring for a good front light (or mount to put you tac light) and one of those blinky red ones on the back, as getting run over at dusk is no fun.
Good luck.