I owned a 930 for 11 years and only recently let it go. While they are the most incredible car, here are some things you should take into account:
1. OEM parts are almost impossible to get now. I needed a new fuel tank and you just can't find them. There are companies out there making parts, but the prices are ridiculous. I think a fuel tank from Pelican Parts [U.S.] was like $650 + shipping... Small stuff is available from Pelican, but other stuff like fuel systems etc. you just can't get.
2. I was spending about $2,500 a year on maintenance, keeping the car as perfect and reliable as possible. This average is based on like doing a $10,000 engine rebuild and average out over other years that may only cost you $800 or so. One thing you must know is that you need to rebuild the engine every 90,000 to 120,000kms.... Usually valve guides and gaskets wear out.
3. You need to drive the car. It does not like to sit around for a few weeks and then be driven. At least once a week is enough. The more you use it, the more it loves you back. Also, the car hates cold starts - cold stops. You can't just drive it down the shops around the corner and turn it off. They need to be driven for a while to operating temperature, before you turn them off.
4. You will need to redo the entire fuel system eventually, as the Unleaded fuel here in Australia is crap and these cars were meant for leaded fuel. There are great places to do the system clean/refurbish/tune for you.
Anyway, that's what comes to mine initially. Don't let me put you off, there is nothing like a 930 at all! I did a Porsche Club Drive from Sydney to Hunter last year and the 930 still has brutish acceleration and top speed to match all the other new cars that were on that drive. I would say a 930 is still better/faster/more fun than any 986 or 987 Boxster and 996 911.
Only through imperfection do you get perfection.