In my short time of actually collecting bikes (and selling half of them off ), I've learned that they're a lot easier to maintain than people think. The PITA internal cables on my BMC were still easier and quicker to finish than a battery relocation kit on my car, for example.

IMNSHO, a bike commuter should be able to nearly tear down & rebuild a bike to its bare frame and back. I'd be satisfied with knowing all the work that can be done with screwdrivers and some Allen wrenches (which is most of what can be done on a modern bike anyway).

As far as internal vs external cable routing goes, one option would be to choose a frame with its cable(s) routed on top of the top tube. My commuter's rear brake cable is like that, so lifting it up isn't hard on the cable at all.