The grading should just be an indication to the condition of the car.
You should always have a guy in japan to check over the car. USS auctions can provide this for you but an even better alternative is to have a dealer you trust over in Japan with a license to attend the auctions.
Your friend probobly knows this though, so nothing new.
I estimate that the car price from the auction would be around 800k JPY if you get the car ready in the UK for about £7k.
It's probobly possible to find a grade 4 for that price, but as I said earlier. A grade 4 could differ from another grade 4 by far.
most grade 4s are generally decent to very good though.
A grade R can also be decent and a grade 3.5 too. Just have to find the right cars
If you plan to keep the car a while I would up your budget a bit.
Or be prepared to do some work to get the car in perfect condition even though it's a grade 4.
Have a look on japaneseusedcars.com
You can find prices from recent auctions there to give you a hint on what you would get.
add about 100000 JPY for FOB price.
Then you can do the calculations yourself to find out about how much the car would cost.
((car price + freight)*tax*VAT)+ESVA+some extra for things you forget to think about
((???+500)*1,1*1,175)+1000+800
Edit: my car was 1880000JPY from the auctions 3-4 years ago.