The 993 Engine Guide
The 993's 3.6 is the last air-cooled engine Porsche put in a 911, and the one with the famous maintenance questions: valve guides, the secondary air system, and the wiring harness recall. None of them are reasons to avoid the car. All of them are reasons to inspect it properly.
Valve guides and the top end
The 993's valve guides wear, and worn guides mean oil consumption and, eventually, a top-end rebuild. A car with a documented top-end rebuild has had the big bill paid. A car without one isn't automatically a problem — leakdown numbers and oil consumption tell the story — but the PPI has to look.
Varioram cars (1996-on) make a little more power. The engine itself is strong; it's the deferred maintenance that costs money.
Secondary air and the harness
US 993s have a secondary air injection system whose ports clog with carbon, which sets a check-engine light and fails smog in strict states. Cleaning the ports is a known job with a known price. The engine wiring harness was recalled for insulation breakdown — a PPI confirms whether the replacement harness is in the car.
Ask for both answers in writing. On a car at this value level, documentation is part of what you're buying.