Depending ont he hub you get, it might be cheaper to buy a budget frame and set up with a derailleur.

As for IGH, I have an 8 speed alfine and it is the bee's knees... and I am a big guy and very hard on parts. It has become my favourite bike over the past year.

What is the droput spacing on your bike? Carvers are mountain bikes, right? SO I would assume the spacing is 135mm, which fits Alfine and Nexus 8. If it is not 135 then you will need something else.

If you've got the dough, the Rohloff Speedhub is still the king of internally geared hubs... although setting up on a bike not designed for it is a little more work.