Flattering while braking – need advice.

Hi guys, I need your help with my brakes. I’m pretty close to the end with my Latin. Short history. I’ve been having a problem with braking from higher speeds for a long time now, that the steering wheel flutters. With slow braking I have the feeling that it slows down quite slightly. So -brams-brams weak-brams-brams weak … etc. You will already know what I mean. So far so good, no drama I think. Order for front new discs and After a visual inspection, however, I had to find out that the front coverings still look pretty good, whereas the rear coverings look quite small. I attached pictures. The rear panes and coverings were made just under 50,000km ago. I never think the front one. That’s what I’m so surprised about. The front coverings look really good for the fact that they already have 140,000 km on the hump. This is the actual question: According to Skandix, I have a brake diameter of 336mm in the front. Unfortunately, I did not find any indication of the diameter on the caliper. Last Mögl. would be measured of course. If I wanted to get a set of ATE brakes and blocks in the bay, unfortunately I find only brakes with the diameter 300mm there. Does this matter which one I take? Specifically it is this:Set 1 or the one hereSet 2 What ha I have rims in winter 17 and in summer 18 inches. Would be great if you would take the time to help with short, since I would like to order the brakes on my own and have them installed by a free familiar. I would of course also be grateful about alternative orderlinks. PS: Auto is a S60II D3 GT 2011 THANK YOU!