Either way you're spending money. For retail purchases a whole bike is cheaper than buying parts retail plus frame retail. Refer to Cyclebums comment wrt fit. That is your starting point. After that the bike should be able to carry the load. Your comments about weight don't make sense because your gear weighs so much more than differences in bike weights. If I was you I'd get seat/handlebar dimensions figured out. It's possible your Mtb tires aren't optimum for touring.