1. You are probably looking at wheels that were intended for this old bike, but you need to make sure because of hub width.
2. If I'm right, you'll have freewheel threads, and you'll need a freewheel, not a cassette.
3. Your other limitation will be how much of a freewheel your derailleur can handle... safe to say whatever's on your existing wheels would work in the same size.
So show us exactly which wheels you're looking at, check the tooth count in your existing freewheel, and then come back here for more advice.