"Rust is not an issue" is one of those Internet myths that you just can't kill. Unfortunately it's also not true. Unless you buy a desert Southwest car you will end up having to deal with some rust- these cars are over twenty years old after all.
They hold up a lot better than, say, a mid-eighties Toyota but they do rust (and I have the bills to prove it). Typical areas that need checking are:
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Kidney bowl area. Expensive repair if done properly, they all need it sooner or later and it's important for the car's structural integrity
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Front suspension pan. Tank sits on it, suspension bolts to it. 'nuff said. Also not a cheap repair but less expensive than the kidney bowls
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Wings rust around the headlamp bowl
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Usual areas that all cars seem to rust - sills, windscreen surrounds etc.
Nothing for these cars is cheap, not even ill-fitting pattern parts. Usually, you're better off buying OEM parts and there are a bunch of companies out there who source OEM parts and can supply at a discount over those that come in Porsche boxes. OEM parts do tend to last a long time, though.
I'm in two minds re PPI, I've had one done when I bought my Targa and the people who did it overlooked some expensive issues (namely, the front suspension pan). If I buy another one then I'd be present during the PPI and crawl underneath the car myself.
Re the G50 vs 915 discussion, I've driven both, I ended up with a G50 car simply because it was the better car but as oldtin says, both are fine. Just don't hurry the changes on the 915 and use the appropriate gearbox oil.
