In most cases the cost is not a great deal more than similar work in a wood frame structure. There are specialty companies in the business of cutting various types of concrete and masonry structures. Depending on the conditions at your home, there may be a need to install a beam or other reenforcing members to compensate for the structural change. Again, this should not be a major problem. If you decide to go with the skylight route, you should choose an operable skylight, one that allows for venting.
Pressure washers are very powerful tools.
Metal roofing can be pressure washed by an expert who knows how to prevent leaks from such work, and how NOT to fall off the roof during the pressure washing. Again, the work must be done by an expert. Once this is done, any damaged shakes should be replaced and the roof treated with a wood preservative. This should be done every 5 years starting when the roof is 2 or 3 years old. It is not necessary to remove all of the lichen and early moss buildup, the wood preservative will kill these organisms, and the dead material will blow off the roof.