Congratulations on the find, Tim!
I also agree that a thorough inspection is mandatory! Keep in mind that the definition of "pre-purchase inspection" varies considerably with who you talk to. To some, a pre-buy is little more than a very thorough preflight inspection, which has little or no value to me in a purchase. A pre-buy can be whatever you request the inspector look at, either more or less detailed. It can take 30 minutes, or 2 weeks to perform!
It doesn't really matter whether you call it a "pre-buy" or an "annual". Done properly, I think they should really be the same inspection, except that the annual gets you a logbook entry that renews the legal airworthiness for another year (when it passes).
Examples:
Two identical aircraft are being inspected side by side, one getting an annual inspection, the other gets a pre-purchase inspection. Both have had the logbooks reviewed and screened. Both have had a close inspection of the exterior noting any discrepancies in fit and finish, dents, repairs, etc. Both have just been flown and the cowlings removed for the engine inspection, starting with the cylinder leakdown ("compression") check. With that out of the way, Both aircraft have all of the inspection panels and fairings removed, and enough of the interior to allow inspection of control cables, pulleys, linkages, etc. Both receive the same detailed inspection of engine, airframe structure, and systems for condition and operation. The bird getting the annual should have various components lubricated (as required for sign-off) during this process, while the pre-buy might not. Both inspections generate a list of discrepancies to be presented to the owner's and the buyer. The pre-buy inspection is completed and that aircraft is buttoned up and may be returned to service on it's current previous annual. No logbook entries are required for a pre-buy. The annual inspection requires the logbooks be signed off, so any downing discrepancies will need to be repaired first. Many owners don't realize that annual inspection and repair are two separate processes. You can have the aircraft inspected without having any repairs made. If no downing discrepancies are found, the aircraft should still be signed off, and remaining discrepancies reported to the owner for later repair.
[NOTE: The repairs required after an annual inspection can be done by another mechanic (who may have less expensive rates or more expertise in a particular area) or even by a non-mechanic under a mechanics direct supervision (such as owner assist). After completion of repairs to the original inspectors satisfaction, the annual is signed off in the logs by that mechanic/inspector.]
Some owners would object to having their aircraft disassembled for a "pre-buy" only (rightly so in some cases, I think). I think most would agree to splitting costs on an annual to make the sale, when they are confident in the condition of their aircraft.
The inspections (without repairs) of both aircraft above should cost about the same. The buyer nearly always pays all costs for the pre-buy. The cost of an annual can sometimes be negotiated and split with the owner on a pro-rated or other agreed upon basis (he gets a fresh annual if the deal falls through), and the owner generally pays for all repairs.
Even though the aircraft you are looking at isn't due for annual for half a year, neither you nor the owner have anything to loose, even if major discrepancies are found during the inspection (the owner eventually pays for those repairs anyway, whether you buy or not). You stand to realize a full year's flying before next annual, piece of mind, and maybe a good mechanic, too.
If I were shopping for a pre-purchase inspector/mechanic, my first requirement is that he/she does NOT know the owner, nor has done any of the recent annuals on this particular aircraft. Second, they should KNOW 150's (being "familiar" with Cessna's is not enough) or at least will have the inspection supervised and signed off by an A&P/IA that knows 150's. There is no substitute for experience and knowledge. Thirdly, the experienced mechanic should be available to become your personal mechanic and do your next annual after you purchase this aircraft (fewer surprises at next annual).
No two mechanics perform these inspections in exactly the same way, and some are more thorough than others, but ALL should meet minimum inspection reguirements as outlined in current aircraft and engine service manuals for that particular aircraft, and the FAR's. What one misses beyond that, another might catch later, or vice versa.
