I bought my 964 four years ago, since then I've done around 8000 miles in it, in that time I've had no major problems with it though recently I've spent around £1000 recently doing a big service and replacing some parts, oil cooler £250, caps and rotors £120 and various other bits and pieces, and like Ian I do this work myself so have probably saved a grand in garage bills.
You have to budget a couple of grand a year in running costs, it may not cost that depending on condition but easily could and more if you have to rebuild the engine or gearbox.
As far as the engine rebuild myth goes, 100,000 is not a mileage that will definatelly require a rebuild, but many have had to have this done due to either bad oil leaks or for other reasons, Mine has had a full top end rebuild and still leaks slightly, I'm not particularly bothered about that just now as it's not getting any worse and it doesn't burn oil.
I would advise you to get it checked by a Porsche specialist, get the engine compressions checked, this should highlight the overall condition of the engine. When I bought mine I did so knowing that one day I may end up with a very large bill if something major went wrong, touch wood so far it hasn't. But these are not cars which can be run on a shoestring budget, not for an extended period of time anyway.
Some people buy these cars and can't really afford to maintain them properly, these are the ones to watch out for, go through the service history carefully.
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