You said you want something to take with you on a bus and someting light to carry upstairs.

If that is going to be the main use (and mainly tarmac riding) I would go for a 16" bike since they are smaller when folded. You need to find a light one since you are going to carry it upstairs.

A light AND cheap bike is not easy, since in the bike world light cost more than heavy.

I suggest you try to find bikes to test ride and test carry. I suggest you look for a medium quality folder second hand. Often lightly used folder can be had for half the price of a new one.

Edit: http://www.bikeforums.net/showthread...ot-wheel-bikes

Also want to say that I am wery happy with my Dahon Curve (bought almost unused at half the price of a new one). I also like my brompton but I find that I am more relaxed when I am out with the less expensive Dahon and also I find the wheels soak up much more of the vibration from the road plus the wide tyres actually feel more stable when riding in mud and sand. Measuring the circumnavigation of the two wheels I am sure the smaller Dahon wheels with BA`s are much the same as the bigger Brompton wheels with slimmer tyres.

The Curve is lighter than the B.