There may be an unseen reason that you want the folder, but if it's not a "must" you can get done much cheaper. Especially since you'll be done with this set-up in 3 years, I'd guess. I lived in Brooklyn for years, walkup brownstone 3rd floor. I put one of these
TopeakŪ Cycling Accessories ? Products - BabySeat? II & BabySeat? II Rack on my old Trek hard-tail mountain bike and did just fine. I liked the "slide off" design, which just left me with a flat rack when the seat was off. Good foot protection, good overall construction. Wife was good riding with the setup, too.
Right now I see a Trek 820 on Ebay for $200 and the Topeak seat w/ rack on Amazon for $179.
--tm