I bought the "old steel frame road bike" and for the second year in a row I feel like I'm probably going to end up with a bike that's far too nice for this challenge. The frame is every bit as rusty as I said, but the components appear to be in great shape. Honestly, I could ride this the way it is. The seller was asking $75 but was kind enough to let me have it for $60, so I should have room in the budget for things like tubes, cables, and bar tape to make it reliable. The tires look like they'll survive 100 km. I'm planning to strip it down to bare metal and repaint it.
Curiously, the components are almost identical to those on the Takara that I entered in last year's edition of the challenge. The front derailleur is different, but rear derailleur, crankset, shifters, brakes, and brake levers are all the same. That's nice because I was really happy with those bits on the Takara.
Now for a bit of research to see if I can figure out what brand and model this is.