The workshop redsuns88 was talking about was Chapman and Chapman (Simon Chapman), in Deshon St where ABS is now. I am not sure if has the same owners. They prepared John Teulan's GTP racer plus cars for Targa.
I have also used Gavin Wood. He is my favorite and charges about $70 for an inspection. He has the diagnostic equiptment to interrogate the ECU. If you don't have this done, its hard to find faults. He has been good to me.
A couple of points to note ( and following on from 45ULT):
DON'T TRUST THE ODOMETER READING These are easily "haircut". If I could have a dollar for every import with 70k on the clock! Jap speedo's have 185km limits so any car with a Nismo, Mines or whatever instrument cluster is also low.
BOOSTED ENGINES OVERHEAT A big radiator is an essential on modified cars. Japan is a cold place so its ok there but when you see pictures of big turbos and fancy KW claims without a nice fat radiator, you expect to replace the head gasket or block.
Most GTR's are in need of major work. They are complex mechanically so any repair job has a lot of dissasembly and re-assembly of other parts to get at the faulty part. This is expensive.
Just about every seller claims the engine was rebuilt in Japan. This is an urban myth like "cheques in the mail".