Hi there 🙂
Regarding headlights, be careful as the law has recently changed. Most aftermarket (upgraded) headlights are no longer legal. If the lens has gone Milky coloured you could always try rubbing tooth paste on it with a tooth brush to try and clear it up a little.
Regarding rust on the doors, I'm in the same boat and gave this on my a6 c5. Will be taking it to a body shop once its back from the auto electrician. But I have seen worse case scenarios as per the pics 😬
Regarding central locking you might find it's the driver's door module, my a6 has just had this replaced but is having further work done regarding be the alarm (auto electrician). The module controls all the central locking. I can let you know the cost of this once my cars back.
Regarding a remap, make sure you go to a professional that has a rolling road for best results. Make sure the cars in good health too.
Cheers
Steve