Get the annual done by a mechanic of your choice. Do not ever ever use the owners current mechanic. Offer to pay a fixed dollar amount, say the equivelant of the cost of a pre purchase inspection, the owner would of course be responsible for the rest of the cost. This way you get an annual for the price of a pre purchase and the owner gets a deal on his annual. Of course any issue / snags that come up will be the owners responsibility to fix and pay for.
I can't stress enough, do not rely on an annual done by the owners current mechanic. Consider it worthless. I've looked at 3 planes in 6 months and all owners and their mechanics swore to god and on their mothers graves that the plane was in A1 condition. Each time an independent inspection revield a number of costly issues.
