I would add one more thing.
The well known achilles heels of the 964 are well listed above (oil leaks, dizzy belts, etc). While you need to pay close attention to these, don't overlook all the regular stuff too. Tyres, brake discs, caliper rebuilds, shockers, routine servicing, paintwork, interior refurb, etc etc. These cars are all circa 15 years old now, and this sort of stuff adds up (ask me how I know!)
One last thing. I know of at least 3 964s which have eaten their starter motors recently (mine included). It rather seems that this is one component which has just about reached the end of its days. Ask them to check it specifically if they can.
Good luck.
John H