Answers

A standard dimmer switch will easily control a traditional incandescent bulb.  If you choose to use a compact fluorescent or LED bulb, you will need to install a "C dot L" dimmer, which is designed to control these lower consumption types of bulbs.  These dimmers are readily available at home depot and lowes.

I would suggest construction adhesive which will attach the trim boards while also providing a capillary break between the block and the trim board so that moisture does not wick into the trim boards.  I would also suggest installing the trim boards at an angle to shed water away from the house.