This is just a small amount of things that may need to be replaced. I really didn't include any sensors or any electrical components. As there is to  much to list . The prices are using OEM or non dealer OEM parts and going to specialty shop or dealership. The prices can vary greatly as area, shop and parts used change price dramatically.

Here is a list of issues from 5 different sources

1. Air suspension pump failure.( If equipped.)  Average cost to repair  . 650 to 1000 dollars.

2  Air suspension. Cost 800 to 1300 per wheel.

3 Transmission conductor plate. Cost up to 1,200

Fuel pump and fuel filter assembly. Cost up to 2,000 depending on many factors

4 oil pan seal cost 500 to 800

5 Valve cover gasket cost  600to 900

6 wheel bearings cost up to 600 each

7 o2 sensors in cost up to 1,000 dollars each.

8 Plugs wires coils,  coils only up to 1400 .this price can be alot more of less depending on many factors. Spark plugs 

9 standard maintenance for 100,000.  Around 800 to 1500 a year.

Honestly unless you have someone who will do work for free or you have the ability, knowledge, tech and tools to do it yourself. If your dead set on getting it have a pre inspection done by a Mercedes specialist or German car indy shop. It cost more but it will help you in the long run.

No

Audi A7 2016

Separate each question into it's own post.

Generally the A7 is a "no" because of the DSG.

It's all these new German cars, all cost many thousands to keep on the road.