Shimano 105 STI levers work fine with MTB rear derailleurs - my tourer has XT at the back - and a Tiagra front, that works with a MTB-style crank/chainring setup.

Beware of your brake setup - you may want cable-actuated discs - road levers won't work with Vs (unless you use diacompe v-brake compatible levers, or a Travel Agent or other device to change the cable pull, or mini-Vs). Also, I think the combination of cantilever brakes and drops makes it too hard to get reliably adequate braking for off-road use.

A number of places offer "dirt drops" which flare out, eg. on-one. Never tried 'em. Also, you can get pretty wide drops from Nitto, wide is good for off-road.

A steerer extender if your bars are low would be advisable. Despite the pic on page 1, I don't think a road-racing posture is ideal for off-road unless you really know what you are doing. Downhills get pretty scary. At least braking from the drops is more secure than from on top.

BTW, after off-roading a bit on a touring bike (nothing too technical), I'm getting a flat-bar MTB. I find that long bar ends are almost as good as drops, you only really lose one hand position (drops, tight tuck) which I hardly ever use anyway.

Have you considered test-riding a cyclocross bike to see if you like the feel? Maybe there is a club near you, someone might let you try an old bike.