Not sure about replacing the heater matrix but you should sell alot of the parts as people still keep these cars running. It also depends on what options you have on the car, that can be retrofitted into others.
Maybe check for other vac leaks in the breather system, the fuel trim could still be adjusting or the throttble body might need a clean and reset with VAG-COM. If you can get it hooked up to VAG-COM then look for any errors, specificly relating to fuel trim or EVAP system as these indicate vac...
I would suggest you register yourself at www.vagcat.com and have a look at the online catalog for parts, it is very comprehensive and hopefully you will be able to find the part you are looking for. If not you can always ask the helpful people over at www.a8parts.co.uk and see what they come up...
What type of A8 do you have?
There is a common fault related to the wires leading to the pad wear sensor being streched over time and becoming faulty, documented with fix here audipages Front Brake Pad Warning Light Troubleshooting
Have it looked at with VAG-COM. Not sure which version you need for your A8 but plenty of people here will tell you it is worth the investment.
You can always buy a cheap cable of ebay and use their evaluation software. Also, there are plenty of people on the forum who offer...
Whens it is restarting it probably is flashing a bluescreen error to quickly to see. When you boot into safe mode apply these options;
Right-click My Computer, and then click Properties.
Click the Advanced tab.
Under Startup and Recovery, click Settings to open the Startup and...
Are you going to get a genuine replacement pipe or an aftermarket job? Only thing I would mention is to thoroughly check the surrounding pipes as I had to replace some of mine shortly after the first split, and it may be cheaper to go all out and by them all together. I have a feeling forge do a...
I have been more than happy with stage 1 from Rtech, and as mentioned before they are thorough in making sure the car is running well beforehand, to the point where they take parts off their own car and put it on yours to test failing bits. For what they charge it is a top service.