Buffers help with case mouth dents and damage, but if you aren’t going to reload for it, all you are doing is saving the paint.
Typically, the fired case comes out hard enough to bounce off and protect your right flank.
Pick up a spare bolt buffer, it’s in the front of the stock and will eventually wear out.
https://www.robertrtg.com/store/pc/HK-G3-BUFFER-COMPLETE-181p2051.htm
The little cocking lever spring that holds the lever down is another part I’d grab, it’s annoying at best when they break.
https://www.robertrtg.com/store/pc/HK-G3-HK33-SPRING-FOR-COCKING-HANDLE-NEW-72p1995.htm