Okay, so at the moment I'm considering these:

• $150 - the Citizen Tokyo that I mentioned before (which does have 16" wheels, not 20") and 6 speeds
• $150 - a Dahon Classic III (also with 16" wheels) with 3 speeds

These are also available on Craigslist, but seem like less-viable candidates (either because they're out of my way, or because my non-existent folding-bike-expert intuition makes me think it wouldn't be as good):

• $100 - an APEX 16" (Buy APEX 16" Folding Bike Shimano 6 Speed - Rack & Fenders at Wholesale Prices)
• $175 - Kuhn Rikon (with only a mountain-bike style rear fender)