Ususally the pre-purchase inspection is the equivalent of an annual. So, you might as well make it an annual. Don't forget to ensure who pays for the odds and ends that will be inevitably found to make the bird as close to 100% perfect.
AOPA has a pretty good standardized contract/purchase agreement on their website. It sets out who pays for what, and so on.
Bottom line - get it as an annual and from a disinterested mechanic.
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