Downtube shifter bosses? Caliper brakes? No offense but I think you may have linked to the wrong image. That's definitely no mountain bike, modified or otherwise.


OP: I think most true touring bikes are well suited to the type of riding you described. Very few will not be able to handle the very light off roading you describe, and they are actually purpose-built for carrying loads.

Some are more suited for off-road than others. The Surly Long haul Trucker has clearance for quite large tires if you feel the need:


No idea how much you want to spend, but the complete can be had for under 1000 dollars or you could build from the frame up.