They sell the wheel with cassette? Sounds a bit strange - is this from Ebay?
Shouldn't be a problem, as long as the number of sprockets matches (e.g. both 10 speed). You may want to shorten the chain to optimise shifting, but I don't think that it would make any difference (going from 27 to 25 largest sprocket may mean slightly more slack when you are in the small / small combination, but who cares? - you are not meant to use the smallest 3 sprockets with the small chain ring anyway).
When you change to biggest / smallest (53/12), the rear derailleur pulley and jockey wheel should line up vertically.
You could also check the wear on the new cluster and your existing drive train using a Rohloff tool. If it is second hand, who knows how worn it is?