You should be looking for at least 100mm of front suspension, a double butted aluminum frame, and Shimano Deore components at a minimum. You want to avoid cheaper straight guage frames because the bicycle will be extremely heavy by comparison, and you want to avoid the lower end 7 and 8 speed (21 and 24 total gears respectively) drive trains in favor of the better 9 speed (27 total gears) speed drive trains.
Are you familiar with Iron Horse Warrior bikes? The one I was looking at was on sale for $279, down from $399. I probably won't ride more than three days a week, and about 50% of the rides will actually be on pavement.