You need a definitive answer, and you'll only get that from HMRC. Any answer that you get on here MIGHT be wrong, and IF it comes to paying VAT (which COULD be UP TO 20% of the ORIGINAL price), then a mistake could be horrendously expensive. Note all of the capitalised possibilities, not a gamble I'd be willing to take without something concrete in writing.Though do also note that if you did get an answer from HMRC, which turned out to be wrong, in law it's your fault, and you end up paying anyway.

Basically, only buy it when it's on 'normal' plates, with road tax. Until then, it's a gamble.

Give HMRC a call, or perhaps DVLA, and ask them, then you'll hopefully get a definitive answer. As has been said, there will almost certainly be some tax to pay, probably based on the current value of the vehicle.