Be very careful how you tread with the price, and how long it will take! £800 leaves no incentive for your mate to finish the job, especially if he's had it for a long time.. for that price you need to put in force something that gets you the car back within 10 days, or expect the whole deal to go tits up, and you lose a mate!
A Mk2 Ive just painted, the £800 at you talk of doesn't even come close to what has been spent on paint, materials. sundry items and a new OE wing, let alone labour etc, good bodywork is horrendously expensive!
Thom