Paying £500 for no gain is nothing. I know people who stumped up 3 PPI's on Porsche inspections (c£1000) that didnt get a green light to purchase. Then they decided to buy from a Porsche main dealer for the 2 year warranty and knowing it had been through a full 111 point inspection and anything from that has been fixed as well as cosmetic respray to sort stone chips and machine polish to remove swirls, all carried out at a Porsche-approved bodyshop. That warranty alone (the best there is) costs a few thousand! They realised that after burning a grand on PPI's. So something to put things in perspective and consider?
My honest advice if you are keen on this car from this supplier is to pay his £200 to have a PPI through Migliore or Graypaul (neither of whom I vouch for as I have never used them or spoken to) and wait for the report. Then you can decide.
From the dealer's perspective, he is gonna expect you to knock him down on price to cover whatever the PPI reveals so he is being reasonable in asking you to cover his costs for transporting the vehicle there and back for a PPI.
Buying these cars sunny-side-up can be a very costly mistake. The AA or RAC inspection is a complete waste of money.