Hi Megjo,
Welcome to the Forum,
Your intended dealer should have a copy of the auction sheet when the car was exported from Japan (probably a while ago).
Even if they didn't import it ! If they cannot provide it, think twice. The document should be part of the documentation trail / package.
If it hasn't just been imported, and it is registered, check out DVLA. Year imported, MOT history, etc.
My due diligence would start with a independent mechanics report unless, of course, you are a mechanic.
Next, you should identify a person able to service / maintain your purchase, should you proceed.
Most official Toyota dealers will not work on grey imports.
Many UK dealers are just dealers. They are not necessarily suitably qualified or equipped to work on the vehicles.
Conversions are not normally cheap, so you must ensure you have a great base vehicle.
Is it undersealed ? Will undersealing it be as part of your purchase contract ?
Is your proposed dealer registered for VAT?
I am sure you can check for outstanding finance, but if it has only just been imported ....
I am also sure you will soon receive other tips.