Mixing road and mountain components is not a problem if you stick with Shimano. Road derailluers work with mountain shifters, and vice versa. You can also feel free to mix road and mountain cassettes, assuming they have the correct number of gears and your chain and rear derailluer can handle the number of teeth. Cranksets can also be switched, again assuming that front and rear derrailuers and chain can handle the change in size. I do not believe you can swap road and mountain shifters and derailluers in the same manner if you are using Sram. Campagnolo makes flat bar controls that work with thier derailluers, but they may be limiting if you are looking for a stump-pulling gear, or trying to keep to a nominal budget. In general, match the shifter and derailluer brand and you should be fine.
As for the handlebar shape, as biker_james pointed out, the advantage of the drop bars is more hand positions. In truth, I doubt most tourists on drop bars (including myself) spend very little time in the drops, but do switch from the tops, to the ramps, to the hoods, and back fairly often. In that case, your barends give essentially the same positions when set up normally. Trying to get "aero" on a touring bike just seems to be a contradiction of terms.
Sean