The errors are predictable, which means they are avoidable.
Buying with the heart before the head
The most common and most expensive mistake is falling for a specific car and then talking yourself past its problems. The air-cooled 911 is an emotional purchase, and the emotion is exactly what good sellers count on. You see the color you wanted, you hear the engine, you sit in it, and suddenly the soft compression numbers and the murky history file feel like details. They are not details. They are the difference between a good purchase and a money pit. The discipline is to fall in love with the idea of owning one, not with the first specific car you see.
Chasing the cheapest example
The second mistake is buying on price. The cheapest SC or 964 in the listings is cheap for a reason, and the reason is almost always deferred maintenance, hidden rust, or a tired engine. A car that costs ten thousand less than the others will frequently cost twenty thousand more once you have made it right. The expensive lesson every air-cooled owner eventually learns is that the cheap car is the costly one. Buy the best example you can afford, not the cheapest one you can find.
Skipping the inspection
The third mistake is waiving the pre-purchase inspection because the car looks good, the seller seems honest, or you are in a hurry. This is covered at length elsewhere, but it bears repeating, because intelligent people skip the PPI constantly and regret it almost as constantly. A few hundred dollars of inspection protects against tens of thousands of hidden problems. There is no version of this car-buying process where skipping the inspection is the smart move.
Ignoring the history file
The fourth mistake is undervaluing documentation. A car with a thick, organized history file of service records, ownership history, and receipts is worth materially more than an identical car with no paperwork, and for good reason. The file tells you the car has been cared for, what has been done, and what has not. A car with no history is a car with no answers, and on an old, complex machine, answers are everything. Buyers fixate on the cosmetics and ignore the folder of paper, which is exactly backward. The paper often matters more than the paint.
Underestimating the running cost
The fifth mistake is budgeting only for the purchase. A buyer who spends every available dollar acquiring the car and leaves nothing for service, insurance, and the inevitable repairs is a buyer who will be selling within two years, usually at a loss, blaming the car for a problem that was really a planning failure. The running cost is real and predictable. Budget for it before you buy, not after the first big bill arrives.
The pattern
Every one of these mistakes comes from the same place, the gap between the romance of owning an air-cooled 911 and the reality of buying a specific thirty or forty year old machine. The romance is real and worth chasing. The reality is a used car that needs to be evaluated with clear eyes. The buyers who do best hold both at once, the love of the idea and the discipline about the particular car. Want the dream. Inspect the reality. They are not in conflict, and keeping both in view is the whole skill.