For your requirements, I would strongly suggest you save up a bit and go for quality which is more important in folders than big-wheelers. Folding bike joints suffer lots of stress and there is loads of "anecdotal" evidence that frame hinges will loosen up over time making the bike useless. I would try and avoid aluminium bikes with frame joints. IMHO they don't have the legs for long time heavy duty use.
You know, that's a really good point, and one that I hadn't considered. Aluminum frames definitely have a much more finite life than steel ones; for a full sized bike I'm not too concerned with that, but I suppose the longevity of aluminum frame joints might be seriously in doubt. I would be riding this bike a couple thousand miles a year.
Another question, while we're talking frame materials (uh-oh) would have to do with ride quality. I've been pleasantly surprised by the ride quality of the full-sized aluminum bikes I've ridden in recent years, but the Downtube's frame is REALLY beefy. Does it have a pretty harsh ride? Can I assume that the steel Swift or a Bike Friday (or Brompton) are considerably smoother? Not the biggest deal, but if there is a significant difference I'll factor that into my decision.
If I were you I would place a Brompton top on my list and find a way to fit discs or hub brakes. Bromptons are pretty reasonable in price for the base models and I assume that the lack of wide gearing will not be a problem for you, given your experience.
My local dealer quotes a Brompton with 6-speed gearing (3-speed internally geared hub by 2-speed Brompton derailer) at $1400. As long as I have a disc up front, a drum should be adequate in back and then I wouldn't have to modify the actual frame. But I'm sure by the time I get a new wheelset and fork, I'm still pushing up near the $2000 mark.
Which makes it (probably) more expensive than a locally made Swift or Bike Friday. I hear a lot of great buzz about the Brompton -- but is there any great advantage to them other than the super-compact fold? Frankly, as long as the bike folds in half that's good enough for me to get it on the train, and I don't mind breaking out the tools to dismantle it for travel.