Hi i have a japan import standard mk5 gti and it has no rust whatsoever despite having nearly 100k miles on it when imported. From what i have read the Japanese treat their cars (all possessions for that matter) very well so it should have been well looked after.
Mine came with the service history so not sure if you can check with VW or not, has it not got any paperwork?
Resale wise i think some people might have reservations with it being an import but if its a good example it will sell. I would say for slightly less than a like for like UK car though.
Have you thought about importing one? There are companies that specialise in it both here in the UK and Japan. Im currently looking to import a mk7 gti and have spoke to TorqueGT and also a Japanese company called Japan car direct. Google them and you can view the auctions, have seen a few MK6 R's on there recently.
Edit: This one is due for auction in a few days with just 30,000km