I do a daily NYC commute on my Brompton 3-speed (M3L). I'm guessing 2 speeds are enough but I would err on the side of more and go for the 3 speed, especially since you've been out of the groove for a while. I think the bigger issue will be carrying the bike up that many stairs - it will feel heavy. Renting would be a great idea to make sure you are up for it. That said, it will be a good upper-body workout - just remember to alternate arms.