4.7 piston with rings are £180 each and the liners are £140 each so that's £2.5k plus some other bits like gaskets, shells and gudgen pins and your looking at £3k. You could swap the engine your self and an engine rebuilder would want £1k in labour. Add a remap for about £750 and your still about half the price of the used 4.7
Of course, you need to do your homework by comparing part numbers on ES to see what you need. If I get a mo today I'll look into it further as you never know, the pistons and liners could be the same and it's the con rods that are shorter, but suspect not.