You don't NEED to convert to a full on 8 speed. But if you get that wheelset from eBay, then you:
A. WILL need an 8 speed cassette
B. Have a spacing issue (minor, but still an issue)

I'd go for taking the wheel to your LBS and having them lace up something cheap to get you rolling. This will undoubtedly cost you MUCH less and then there's no spacing issue at all.