Have just come by a ss 2nd hand to try it out. Having been pleasantly surprissed at how good it is will keep it. It did come with, to me, a fairly hefty wheelset in the form of Planet X Monolith hubs, unknown freewheel and Planet X LMF rims. Not the lightest but looks good enough gear. Have option of keeping this on bike or sticking a cog on to a normal hub on a set of Mavic 223/LX wheels have spare and flogging the Planet X gear.
Probably won't notice any difference whatever I do. Any pointers?

Cog on a normal hub seems to work pretty well for me. A single speed specific rear wheel should be stronger, due to lack of dishing, but it sounds like the SS specific wheelset is going to be a bit heavy for general XC duties (assuming that's what you want to do with the bike, as you haven't said).

It really depends on what you want, something very strong but heavy, or lighter but not nearly so strong. The chances are, unless you are weight obsessive, you'll not notice either way.