I had considered using a Linksys box with the Sveasoft firmware and that would also be a cheaper option, but I'd like to be able to have just one box if possible.

The only product I've found that comes close to doing what I want is the ZyXEL Prestige 660HW which has everything but WDS support, so I'd need to replace the airport in the office with a regular wireless-to-ethernet bridge. I've not had any experience of ZyXEL kit before - does anyone have any experience of using any of their products? From reading over the incredibly detailed manual it seems to have a very high spec for a £50 router.