Definitely don't rule out aftermarket upgraded items from pioneer, JVC etc.
Have a look at this regarding B&o speakers:
https://www.audiownersclub.com/forums/topic/25250-b-o-is-it-genuine-or-just-a-rip-off/
Nice one, sounds like the A6 is in good hands. Whilst the front is off for the cambelt and water pump it might be worth replacing the lower pulley as mine nearly fell off! And the thermostat whilst he has easy access.
I have to make you aware of the legalities of it as a staff member. I fully understand why you went down the remap route.
This is the thing, regardless of how old audits become the servicing is always expensive.
Definitely worth speaking to autologics or Audi retrofits regarding android auto to give you an idea of cost.
Check YouTube for buyers guides.
Regarding quietness all the cars will have the same sound deadening etc.
Check this thread regarding B&O
https://www.audiownersclub.com/forums/topic/25250-b-o-is-it-genuine-or-just-a-rip-off/