There's no need to upgrade as such anyway. When a part breaks, then replace it with an upgrade part
For my v4 Kraton I've got the m2c chassis and braces as I plan on trying some big jumps. If/when the shock towers bend, then I'll upgrade them to m2c
For my Talion all I got is GPM chassis braces as it will just be a general basher/speed truggy
I also bought the GPM suspension hangers for both, that's mainly just to add a bit of bling, and centre diffs for both