Bose would be easier to retrofit but I'd recommend you speak to autologics or Audi retrofits as members have used them for retrofits and been happy with the results.
I'd recommend a diagnostic check to see if any fault codes flag up. As it could be an issue with the mad sensor, lambda sensor, cam sensor. What's the oil consumption like?
On the A6 C5 the egr cooler has been known to put oil into the coolant system. I appreciate its a different engine though.
As you bought it ROM a dealer within 30 days you're entitled to a full refund.
Have you had a diagnostic check to see if any fault codes flag up?
Has the cabin had an ingress of water at all?
The ECU could be at fault, you could get it sent away to be tested.
Water ingress is caused by blocked battery drain holes under the scuttle panel or cracked pollen filter housing. I've had both issues.
Yes good point. But bare in mind instrument clusters can fail.
I'd definitely recommend a diagnostic check to see if any fault codes flag up.
Welcome to the forum you'll find the members on here are a friendly and helpful bunch. I believe coding started coming in from 2007. So I believe your battery will need coding.