Here's my 2 cents...
997 GT3s are fairly robust, if you need a warranty for piece of mind then that's fine, but I'd be interested to know the percentage of GT3 owners of cars this age have a Porsche warranty. Personally I wouldn't restrict the search to just the OPC network as you could needlessly miss out on a decent car
I would agree that buckets are worthwhile but don't let that stop you from only looking for a Clubsport... But I would say that... Mine isn't a Clubsport, but I have Recaro Pole Positions which are very similar seats as the 'Porsche" versions fitted to the Clubsport (Recaro make them). I have the XL size seats which are slightly bigger than regular, but they are still a very comfy fit. Like Colin says you really need to try for size to see if they suit you
If you're not afraid of doing a few mods, then worthy upgrades you might want to look at are lighter alloy wheels (like OZ), as they make the car feel lighter and improve the handling somewhat (as this mod would do for any car). Also a centre muffler bypass is a fun mod too (although this can be a pain if you plan to do track days anywhere other than the Nordschleife, Spa or Silverstone, due to track day noise limits)
If you are doing track days then fitting 245 section tyres on the front will help reduce understeer. Also avoid the factory spec geometry as this is supposedly 'safe' and pro-understeer. Suggest a visit to Centre Gravity for a more neutral balanced geometry setting, which will provide noticeable benefits for road as well as track driving
If you're looking for a completely standard car, then finding one with a OPC warranty should in theory be a good guarantee that the car is indeed standard. It may have been modded in the past but most mods such as those described above are reversible. So if you find a car on your travels that has been modded, don't ignore it as it should be fairly straight forward to return to stock. Unless you're talking cars with engine modifications but that would be very rare...
I think people tend to hold on to 996/997 GT3s these days (unless a change in circumstances forces a sale), which is why there are relatively few for sale (although they weren't produced in huge numbers in the first place). I've had mine for 8 years - it had 3 owners in its first 4 years (I put that down to 'new toy" syndrome experienced by most new sports cars), so don't necessarily be put off by a car that has a high number of previous owners. What is 'high" is of course debatable...
Assuming you get out on a test drive, listen out for a creaking noise from the front end when going from lock to lock, this will most likely be worn DU bushes in the Bilsteins which will need a refurb
Note also that the factory LSD is made of chocolate, so look for evidence that this has been replaced or upgraded to either the Motorsport or Guards LSD
I can't think of much else. Coolant pipes are a major issue across the pond but seems to be less of an issue over here in the UK, but seriously tracked cars should probably benefit from having the coolant pipes welded or pinned in (the factory glues can work themselves loose after sustained heat cycles apparently)
Goes without saying that you should look for signs the car has been looked after and maintained regularly. Over-maintenance isn't a bad thing too especially for a car that has seen the track, so look for evidence of extra oil changes between main services for instance
The above is based on my experience of the 997.1 GT3 specifically, but probably applies as much to the 997.2 and 996 models
Good luck with the search :thumb: