Ok in terms of extending the network, you are right, to make it "future proof" you should use a switch. Tho the way I did it as I was pulling floorboards up anyway is to cable into every room, and only put sockets in the ones I need for now, all the cables lead back to the router, and if I ever need to a switch will be installed there. Not much use if I decide to move the router I guess but as its by the phone socket shouldn't happen.
In terms of the USB, I have seen routers with support for USB hard drives AND Printers, its possible yours may be capable of that, best way is to test. If it isn't then assuming you definitely want to go that route, then assuming your router is linux based (Which every one Ive owned is/was) then you could build your own kernel for it, there are plenty of guides on how to do such things online, though from experience I would say it can be a pain as for me at least there is always some functionality you forget. From that point of view it may be easier to attach it to one of the clients, or even if you want it that badly, build your own PC from scratch, looking for low power consumption etc, and put it in a cupboard somewhere
Hope this helps, and apologies for any typos, Ive been at the pub with soem old school friends