Not sure why he needs v5 or mot. Surely he will have to re register in aus like foreign vehicles here do within 6 months. Re drive to soton, why isn't he using a car transport firm, agree should pay fuel costs/ return costs unless you are near by (poss even your time.) what's the intention to sell letter about? Surely receipt better upon funds transfer. Be careful with funds transfers, they can be reversed. Cash is your friend, or insist transfer into an account (set one up for thus purpose)and withdrawal of funds and deposit into your own account before you release.
It may be genuine, but could be major hassle.....just keeping the deck loaded your way, keep at it (but don't stop advertising car, uk sale easier!)
By 5 bob worth!
Ed