Since you are building up the frame, you COULD put a disc fork on it. Since you already know the advantages of discs, an Avid BB7 (Road or MTB depending on levers), might be good for you. Even at your light weight, the extra stopping power on the front might come in handy on steep downhills. As far as V/canti, I prefer V brake with Brake Booster. I have road style levers on my touring bike, and so use a 'gasp' Travel Agent. In spite of what you may hear, it was not hard to set up, and works very well. Gives me fine modulation and braking power.

For cranks, I use XT Hollowtech with 22/35/48 rings. External bearings, with A520 pedals (spd single sided). XTR RD with 11-34 SRAM cassette. Plenty low gears for towing my trailer uphill, and plenty of stopping power for the downhills.

I won't try to talk you into trying a Brooks, but you might consider a Thudbuster seatpost. Takes a lot of the jarring out of rough roads.