Originally Posted by
MrInitialMan
For the bottom bracket--or whatever it would be called in this contraption--do I need to get something from a bicycle parts dealer, or would a short pipe with a couple of bearings tapped in do the trick?
You need something that's compatible with the chainset and cranks is the thing. Bottom bracket shells from bicycle tubing suppliers don't cost very much and you can get a threaded one (for much less than the taps would cost to make it). Threaded bottom brackets are the most ubiquitous, best standardised and are reliable.
It also might be an idea to cannibalize an old bike frame or two also for some of the parts since other bits might come in useful.