I am going through a suspension overhaul on my F31 340i at the moment as well.
There is a lot of great info especially from Farkle and others in quite a lot of different threads.
As said already it completely depends on your goals.
I have decided to go down the B14 coilover route, with M4 front arms.
One thing I have noticed with searching the forum is there is quite a lot of old and incorrect information around the M4 arms in some of the threads so watch out for that.
Here are the arms:
https://williamsperformance.co.uk/co...ts-upgrade-kit
I am now debating what top mounts to go for it which is a whole other rabbit hole.
Millway seem to be the way to go for ultimate adjustability and quality.
Street versions for the front for a bit more comfort with the poly bushes?
https://www.millway.se/camber-plates...0-f31-f32.html
But unsure if these are worth it when I will gain the extra camber with the M4 Arms already...
As for rears, I am currently debating standard top mounts vs solid Millway:
https://www.millway.se/upper-top-mou...-g-series.html
And then there are rear toe links to consider also!
https://www.millway.se/control-arm-t...1-f32-f33.html
Goodluck & let us know what you decide