Brake Disc Replacement and DPF "Flexor" Issue

[djmitch]

Having recently bought my first (used) BMW a couple of months ago I am hoping a few of you with more experience might be able to help with this.
To cut a long story short, both my local garage and the local BMW dealership tell me that I need four new brake discs. The thicknesses were 18.1mm at the front and 20.1 at the back (minimum is 18.4/22.4). I went back to the independent dealer who sold me the car since it comes with a (admittedly limited) 6 month warranty. He inspected the discs visually and claimed they were good for another 20,000km. He paid for DEKRA here in Germany (this is an independent body who do the German TuV or UK MOT equivalent) to do an overall assessment of the car to reassure me, but they didn't measure the discs either. The mechanic likewise had a look and a feel and suggested that I get them done when the pads wear down. My instinct is that, since the rear pads are quite worn anyway, that the rear discs ought to be replaced immediately. 2.3mm under seems like a lot. But if I understand the notion of a minimum disc thickness correctly, the DEKRA guy and the seller are correct to suggest the front discs are still good until the pads wear down since they were presumably above the minimum when the pads were fitted.

The second issue with the car which I believe is causing some louder than usual engine noise especially starting from cold and stopping is a slightly torn flexor between the DPF and the main exhaust shaft (not the best picture , but you can see it coming away around the top). Again BMW and the local garage say replace it, but the seller of the car just says that since the exhaust system is not leaking there is nothing to worry about. Subjectively I'm not too happy waiting around for it to tear further and fail before getting it fixed. All this difference of opinion is a bit overwhelming to the novice so some I'd like to hear your thoughts before putting my hand (deeply) in my pocket. BMW quote 1800 euros to do the lot, of course elsewhere it will be cheaper. Cheers!