Looks like you're on the way to something interesting. Here's my two cents:
Cassette: These guys might be able to help you whip something custom built for your purpose:
http://sheldonbrown.com/harris/index.html
They can also help you find the right brakes, bags, lights and fenders (maybe). Ruffy Tuffy's are fantastic. I commute with them over a hell road of loose gravel and pot holes and trust them completely.