This is a fairly straightforward conversion as far as I know.
The wheel shouldn't need dismantling and rebuilding, and I would suggest the easiest route would be to send your wheel to St John St Cycles rather than to Germany. Phone them up and ask them for a quote for the job, I doubt if it will be very expensive.
All they need to do is drain the oil, remove the gear change mech (internal or external)and seal, and the current end-cap and seal, fit a new seal and the disc compatible end-cap, refit the gear change mech, and refill with oil, probably less than an hour's work.
You could probably do it yourself if you can get the hub end-cap somewhere.
I don't know how much the hub end-cap will cost though.
