what do you mean by no logbook ?
he's lost the V5 document ?
if so what is he going to send to the DVLA to register the car to you. You've got the new keepers supplment, but that means jack if he doesnt contact the DVLA and register the car to you.
The possibilities of how this could go wrong are endless.
And you want to know why he's hiding the V5 from you ? because if you get the number on the V5 document, you can look up online exactly what the car failed its MOT on. Yet he mysteriously cant be bothered to MOT it....
so not only have you got no idea how roadworthy it is, but you've got a suspicion that its not, and he's hidden the evidence that you could have used to find out how bad it was ! omg !