if you just want your bike for simple single track, plus whatever pavement lies in between, I think a conventional touring bike will do you just fine. I've done quite a bit of off-road riding with my loaded Trek 620, and it's never given me much of a problem. Just mount wider tires than usual, and go slow if the terrain calls for it. The real fun is how you set it up, dirt drop handlebars and a thin 29er tire (or fat cyclocross, depending on your viewpoint) can make it a pretty capable mountain bike, if you go gingerly.