My 2p
Lets say you bought a good 996 (15-18k) with 75,000 miles on it. You then over the first year spend another 3-5 k on getting it tip top which many do. Then spend a few quid each year on general running costs. By 100,000 miles (5 years ownership for many) you are into the car for probably 22-23k.
At this point you have a choice.
Do no more than continue to look after it and drive it. You could happily get another 10 years with no real trouble!
Sell it for 15-18k (lets say prices stay stable) you have had 5 years of enjoyment for comparatively little money.
Rebuild the engine for total peace of mind. Keep it then for ever and enjoy it. Yes it then owes you over 30k and worth 20k but that's less than the depreciation on pretty much any new car, let alone a fun one.
Life is all about choices and what you are most comfortable with
I would never have bought a 996 if aircooled ones hadn't gone mental. Now I find I actually prefer driving it to the aircooled ones I have tried. It's modern enough for there to be few compromises yet old enough to remind you what "feel" was. I'm even becoming fond of how it looks :thumbs:
Imagine how much it would cost to get a 25+ year old aircooled car in tip top condition! They may have increased many times in value but their condition hasn't improved.