Urgent import duty/registration advice

I'm after advise on a possible purchase which is a little complicated and certainly out of the ordinary (and my comfort zone).

A guy has worked in Japan, has owned a car for years over there and then moves back to the uk, i have found that provided he was moving here and didn't intend to sell in the following 12months that he will have been able to avoid import duty on the car.

Now say he is now looking to sell, say >12months (i'm checking this with him) have passed but the car was initially used on its old foreign plates and then layed up and so is still NOT uk registered.

Now here are the questions....

If I buy it is there anyway I can get stung for the duty given its unregistered status? (I know for example a car bought from the dock import auctions is subject to duty and vat as they are brought in specifically to trade and not as a private owner is in this case)

He is claiming to have the relevant proof of ownership docs required to register it (needs uk mot and to be insured via the chassis no. too) but what are these (Japanese reg document?) and could I go with them to get a V5 in my name........if not, if the owner and I both went together could we sort out a V5 in my name? (i'm planning on calling the DVLA but not 100% hopeful of finding someone totally knowledgable on the subject and even if I do i'm not so sure the local office won't be difficult)

Basically can I be left with a unregisterable car and/or a large duty/vat bill!!

Obviously for me the way forward is for him to do all the registration stuff then wait for the V5 in his name before selling to me in the normal way, but time is of the essence as he is leaving the country very soon.

I think I'm as little mad for contemplating it but the guy seems very genuine etc.....