I would buy a Dahon Boardwalk D8 for about $500 new, or used on Craigslist for half that. It has an upright riding position, a low step over, gear range suitable for your ride, weighs about 28 lb, and folds up. The folding up is important: in a narrow steep stairwell, it is easier to carry something that is 28 lb and the size of a suitcase, than something that is 28 lb and the size of a bicycle. You could even figure out a way to attach a shoulder strap, just while you are carrying it.