One option you might want to consider: if the aircraft is in need of an annual, or will be soon, you might work a deal with the seller to hire a mechanic of your choice to do the pre-buy and also at the same conduct a full annual, and split the cost with the seller. That way, if you like the results from the inspection and close the purchase, you not only have a good pre-buy verification, but you're also good for a full year on your annual, all for half the normal cost of doing both. It's good for the seller too, not only to help make the sale, but also if the sale does not go through, he's got an airplane with a fresh annual for half the usual cost, and it's good from a marketing/sales perspective in finding the next buyer ... it can be a win-win for both parties.
I did that when I bought my Cherokee ... it worked out very well for me, cost me a total of $500.
Just make sure that the IA/pre-buy inspector knows and acknowledges that he's working for both seller and buyer (get it in writing), and make sure you get to pick the IA.