If I've got a back wheel with an 8speed cassette

they're not particularly nice rims anyway - i would only keep them for convenience.

if a track hub is what's necessarry i might as well go the whole hog.

cheers!

Surly make convertor things to change cassette hubs into fixed. Totally pointless and overpriced though in my opinion.

Indeed just buy yourself a good rear wheel, will be an investment lasting you through a few bikes.