What stage are you at in the purchase?
What does your solicitor say?
What is your plan B?
As others have pointed out, the lead time for an FTTP install *could* be long so does that mean you won't be in a position to exchange contracts until it is in place to test?
I see that you're prepared to pay up front so the current owners are covered but what happens if they give up waiting and sell to someone else in the meantime?
Can't you just ask the current owners to place an order to see what happens, and then cancel it if it is accepted? I don't see the need to test it when installed as will either work at full speed or not at all.