I have a Surly Cross-check. Not the lightest bike in the world but it suits. I can put 28 tires on it and have room for a fender and run it on dirt roads or I can use 23's on it and use it on the road. You can also put a rack on it. I've done fire trails and lots of road riding. Its steel so its heavy but its also a nice ride.
The frame sells for around 400 and built up I think I've seen them for around 800. Some bike shops carry them. I ordered mine on-line at aebike but you should try one before you buy.