I'm not a professional but I maintained my own bikes for the last 15 years so I'll take a shot at it.
1. To get a set of reasonable quality wheels that are setup to accept disc brakes will cost at least $100 for the rims, $25-$50 for each of the discs, and however much you're willing to spend for the sprocket assembly.
2. No. In fact I have found that it is much easier to change wheels with disc brakes than with traditional brakes.
3. You can do it with no tools if the rims have a quick-release nut, otherwise it would only take a couple of adjustable wrenches. Another option is to take rims that don't have the quick-release into a bike shop and they can install it on them for a reasonable cost.
4. The weight of the frame might affect how fast the tires deflate, but the bigger issue would be how much you weigh.
5. No, as long as you have the same size discs and the same number of gears in the back, you can just change wheels and be on your way.
6. There is a HUGE difference between riding on a road with dirt tires and riding on the same road with road tires. It will be significantly easier to use road tires on a road than try to use dirt tires on a road. The question (to me anyways) is whether or not the benefit is worth the price.