If you can find an older, used, good-quality mountain bike with a steel frame, no suspension fork, and maybe Shimano's Deore-series components, go for it. You can always slap on some hybrid-width slick tires if you feel the stock tires are too slow.
Pay attention to frame size too. For a rough frame-size guideline, subtract 10-12 inches from your slacks inseam. I wear 32" jeans, I subtract 10-12 inches from 32, I get a frame-size range of 20 to 22 inches. For road riding, I can do the taller end of the range; for off-road, the shorter frame may be better for crotch clearance during "unexpected manuevers." 😱
If you're going to have to get a Wally-world bike, buy it used from a bike shop. At least then you can expect it to be safely assembled and tuned to whatever potential it may have. But if you can get an older Deore-level, shop-quality bike, it may pay in the long run when you don't end up buying a new rear wheel because the bearings disintegrated, etc etc.
I'm following my own advice, I bought a used GT Bravado LE, probably a '00 model.
