You're taking a big chance with that trade. But, with £100 cash thrown in on the deal, you may get what you need to fix it up right there.
Personally, Id want to see the quad he has up for offer in person, test the compression, check the frame for bends, look for bad bearings in the rear end, etc... If everything checks out and you are willing to take a chance on it...its up to you. I don't think Id make the trade unless he was willing to put more cash with the deal. Its too easy to wind up with an engine that has a broken piston skirt, cracked cases, etc. Then what was supposed to be a simple top-end jobhas turned into a full-on engine rebuild.