you need a Dahon Cadenza!!
I had pretty similar requirements - needed a folder to get up and down stairs and store in the flat, but my commute is just shy of a 20 mile round-trip.
I was quickly steered away from small-wheeled folding bikes in the shop as I was told they weren't really suitable for such a long commute - they suffer much more wear-and-tear for a equivalent journey than a full size bike and I was told I'd be constantly replacing parts.
The Cadenza is a decent road-equipped hybrid which happens to fold in half. It's a bit bulky for getting up and down stairs, may depend on how narrow your staircase is - I manage on our narrow Victorian staircase but it's a lug.