While it's counter intuative to go to a shop who sell shiny new stuff, Steven or Wayne in CyclePlus may have an old scrapper from a customer upgrade? It is worth asking and I am sure Wayne could dress the wheel with the right components.
It may not be an option but at the cost of a phone call is worth a go surely?
Hey Fiona, just match exactly what you have so long as your old wheel can accommodate the cassette. On your road bike on its current cassette there should be a cap on the very end that has the number/make etc. It's the outer ring - on your road bike that will tell you the cassette you need for an exact fit. Then again, you may get an old wheel that exactly matches what you already have and then super - no need to fit a new cassette, so hold on til you get that old wheel! BTW - shed loads of wheels on adverts.ie from 20 quid up. Just get the right size!