I would have said £500 tops in labour to swap them over.

Alot cheaper than having your car sprayed + buying the bits of the GTI.

Buy the GTi CHEAP and swap the lumps.

Depending how bad the head is on the gti it may be worth selling your car, buying the gti and having the work done, £500-£600?

If the lump in the gti was ok before, it would now have a new belt, oil, fluids, etc as part of the repair.