carbon - For get it, go with steel. You might even look at the vintage board for advice on a 1970s frame.
Consider an internally geared hub, much less maintenance than a derailer. http://sheldonbrown.com/harris/hubs-internal.html
Consider Phil Wood hubs and BB.
I like those saddles with springs on the back, and handle bars that have me sitting upright.
You might also consider a folding bike like xooter swift (ask for steel frame)
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.