I TRIED to do mine with the head off, but failed spectacularly as I couldn't get enough force on the valve springs to expose the stem seals. I didn't have a decent work bench/area though, and only a poxy little valve compressor.
If I did it again, I would take the head off, and send it to a decent engineers workshop and let them worry about it, plus you prob might need the valve guides doing anyway and you can't do that at home. Some people have put new seals in, for them to only last a few weeks due to excessive movement in the guides. Plus you could have the head skimmed, which is also advisable.
You could always buy a second KR head of ebay (very cheap) and have it reconditioned, then swap the heads over. That way you're cars not off the road for long.
I honestly don't think the engineers would charge that much.
EDIT: If you do try yourself, get the biggest badass valve spring compressor you can find and test it fits beforehand. Plus sort out a good work bench with vices/clamps etc.