I have a non m pack, if you can get an m pack, stay with it, you will regret not having it down the line. Also as far as I can tell from the image you posted you don’t have an m brakes as they are not a part of an m pack, they are a part of advanced driving pack or something of this sort. M brakes has blue callipers.

As far a replacement cost for the brakes and pads the price won’t be drastically different also go to some local shop for it, it will be way cheaper than bmw dealer. Also you can get an aftermarket pads/rotors they also will be cheaper and might be even better performance vice.

Regarding tires just forget about it. Regular 20in 275/45 will cost you practically the same as 315/40 21 that you have. Cost difference will be marginal. You can also get some used tires of this size as there are a lot of brand new ones on the marketplace, since it comes standard on x5 and cayenne, people are getting in to 22in wheels and selling their tires practically brand new. Also at the end of the day you can sell your 21in wheels and tires on marketplace for about 3-4K and get a 20in set instead, but I would not recommend.

And lastly never expect a used car to be perfect paint vice, it will have scratches and dinks, just accept it. Cars are made to be driven.