the advice on used german cars are, be prepared to pay double, because it will cost the same to get fix and maintain the car too.

make sure you see the service history and inspect that stuff has not been added there for fun, and assume that the car was not maintained for selling (relying you to change the oils/regular parts).

140k, because after the warranty runs out things will start to break.

Im not sure how secure iDrive is, but you can downplay the cost of the car if there is no service booklet, because its not like you can cross reference iDrive with the service center compared with the booklet.

The service booklet and car manual are 2 items that contribute to the cost of the 2nd hand car, so if they arent there, downplay the price but make sure you reference from where and why since they would have the attitude that if you dont pay their price they will just find another buyer.

that would be partially right, to explain the attitude "we use a more advanced system that is secure against tampering" to be more exact, to maintain a german car you need to buy their expensive equipment as other things even the diagnostic computer which you plugin to the same DB port has to be proprietary. That does not mean someone can be unscrupulous at a repair center and claim to have done service on your car and not actually carry it out, which is why they tend to frown on dashcams and any other sort of recording stuff that they would just cover or remove it prior. Basically both the owners and the center would say "we are the future" in this sense. However a regulatory body may not car and simply ask for the booklet which is something that very rarely occurs but is procedural, hence why the booklets and manuals are considered significant for a used car should you service the car independently for the manual containing both user and servicing instructions.

In europe, forcing you to have to service at their place only is illegal if you do not have the choice to service yourself or at an independent place. Tools they can control, but not your right to do it and that is why the manual is important too, should say 10 years down the line a mechanic works on it, even if its a german service center they might not be versed with older cars and have to look it up. Though with the advent of the internet and internal manuals, that does not mean one should rely on them as that too could be down on the day of the service, just like here when you go to pos malaysia and its down because of ransomware so no work gets done.

not much about cars, more on procedure and legal. Its easy to figure out if there is a law that covers something since the law tends to follow precise intentions.

not stock, but you can get an aftermarket one. Scotty kilmer suggested a decent one for about $100 which is about rm 400+ which has its own screen powered by USB, and a wireless camera which he shows how to hook it up. You can get someone to do the wiring and pull from regular/reverse power and just attach it to number plate.

Stock camera depends as part of it involves wiring. The stock camera could either be wired or wireless and the mounting for where it should be also has to be considered (on some cars you'll see a cut out where a part of the body looks like it was meant to be removed. Also the camera itself for the car is likely to cost quite a bit as well. So yes you can add the stock camera but you'll have to find someone to do it.