+1 on buying relatively cheap to start (though test-riding, of course) as after 4 months of commuting, I have learned a LOT about what I specifically need in a bike, and have ordered another one.

As for the Metro here in DC, I have been asked twice in 4+ months of riding to "bag my bike". I have a mesh sports ball bag that mostly covers my Dahon Speed P8, and no-one has hassled me about that. Again, it was only twice in 4+ months that Metro folks even said anything at all, and I am seen by Metro drivers and others every day, multiple times per day, and they usually never say a word. But have a spare cheapy, lightweight bag just in case.

I don't think you'll find a mega-small fold is all that important. It sure helps, but even the Dahon's relatively large fold is perfectly fine for Metro. Smaller than baby strollers that abound, and people give way for those. The fact my bike is folded seems to appease most people.

I can say that weight is a key factor, especially as I've been building leg strength. My new bike will be about 5 pounds lighter, and I look forward to this. THere's lots of up and down riding from DC into MD, so you will be pulling steady grades.

Don't forget tires--the Dahon came stock with Big Apple 2". They are heavy, but really nice on the crap road surface and occasional sidewalk stretch. Consider something wide-ish with puncture protection. The Big Apples have ridden over bottle and car glass with no ill effect.