I'm not sure if hydraulic controls would fit on the "flat" section of a drop bar; I believe that drop and flat bars have different diameters. I don't own a drop bar bike so I'm not sure - others here can confirm or correct my assumption. While drop bars are traditionally great for touring there are alternatives. I have trekking bars on one of my bikes. The flat bar controls fit just fine but required a longer stem and some creative cable routing:



Another option is using bar ends; they come in all shapes and sizes, even ones that mimic drop bars in their design. Personally I think trekking bars are the next best thing to drops and they will certainly allow fitting of hydraulic brake levers. That being said, many of today's mechanical disc brakes, like Shimano BR-M416 or Avid BB7, work very well, are easy to maintain, and will work with drop bar levers.

As for the Soma build - I would have a reputable LBS build up your bike if you lack the knowledge (or cycling buddies who will work for beer). They would hopefully be able to source out the necessary parts more cheaply than you could, unless you're an Ebay wizard. The difference between 105 and Deore (both Shimano) components is that the former is for road bikes and the latter for MTBs. Road bike components typically have closer ratios so that riders can find the "sweet spot" that provides the most efficient propulsion. MTB components allow for climbing in the (numerically) low gears and speed in the high ones. That's why MTB components are great for touring; you can climb while fully loaded with panniers and gear. Check out Shimano's website at http://bike.shimano.com/. You will see a list of both road and MTB components ranging from best (and most expensive) down to entry-level components. As for cost of the build you will have to discuss that with your LBS; it will depend on their rates and the component level you choose. Be sure to shop around and try to get some word of mouth from other customers.