Rebuilding a GQ TD42 ???

Has anybody been through this exercise. My son's TD42 is nearing 600,000klms (yes, that much!) and not surprisingly is getting a bit tired, though still running OK. The rest of the vehicle is still good mechanically.

He's considering stripping it down and doing a recondition/rebuild sometime soon. I've done a few rebuilds on older petrol engines over the years, but have no idea about the costs involved in doing a Japanese diesel. Has anyone got any feedback on costs of parts etc in rebuilding a TD42? Obviously a lot depends on what parts need replacing, but any feedback at all would be appreciated. (My son and I are quite capable of doing the actual disassembly/reassembly ourselves, but will obviously need to send any bits off to have re-machining etc done.)
Thanks in advance for any feedback,
Cheers