Hi BangoO,
Regarding noise, comfort, power and space:
1) I think all the Boxsters, Caymans and Carreras are noisy because, to get the best performance, they all need to be revved quite hard. That's fine when you're in the mood for that or if you are at low speed on semi-fast roads around town. However, you're right, if it's all the time or on high speed journeys on the motorway, these cars can be noisy and this can become bothersome over long journeys of several hours.
I therefore have a suggestion for you that you may not have considered. How about a 996TT? If you look around, you should be able to find a well-optioned one in good condition within your budget. The key thing for you is that you will get a car that produces far more power and torque at much lower engine speeds. It means that the engine noise (at the same speed as you would normally drive your Cayman) will be much lower.
2) Comfort? Yes, I think the 996TT is a good car for long journeys when you need some comfort. It's easy to drive with a quite soft suspension so you won't feel like it belongs only on the track.
3) Regarding power, you will notice the much greater power. The Mk2 996TT has 420bhp. That's a big increase on your Cayman S (295bhp?). It produces a mountain of torque which you will really appreciate on acceleration out of corners or on straight roads where you floor the throttle.
4) Regarding space, the 911 is great because you can (a) push the drivers and front passengers seats far back - so you have loads of legroom (which you do not have as much of in a Boxster or Cayman), or (b) alternatively, you can use the area behind the front seats for storing bags etc. Very handy!
I would add another factor. Since you only buy pre-owned cars, you clearly are a person who wants a great car but without the same depreciation cost. This is the smart way to own and run a Porsche. IMO a good 996TT has done much of its initial, heavy depreciation already. So the cost to you in depreciation - after you purchase it - should be much less.
A last factor - TT owners who buy new often have the biggest budget to spend on options. You are more likely to find a fully loaded 996TT than a 986 or 996 C2 for example.
So I know you have been considering the advantages and disadvantages of the Cayman S and 997 C2S BUT I really think you should look a little more widely.
I realise that a 996TT within your budget will have more mileage than a 997 or Cayman BUT, it is in great condition with a full Porsche service history, you should be safe with worry-free ownership. If anything the 996TT engine is built to handle high mileage better!
Just a suggestion which hopefully you will find useful