Usually it is recommended by many standards to keep PF at 0.92 instead of "Unity". In PF at unity it may easily slide to the capacitive "risky" side and that is the reason why 0.92 is considered a normal level.
By the way, why wouldn't you use a PF corrector with several low...
There is another reason for grounding the neutral:
Since the earth, with all its variations IS the basic "0" from which the potentials are measured, grounding the neutrals create the same "0" for all measurements around. On small distances the earth's potentiality does not...
I heard that european companies such as Merlin Gerin, Siemens or Klockner Mueller have their panel planning software that might do the job for you, and check the drawing for the correct functionning.
It depends on your loads. But if you don't know them, or if you want to play safe, just make the sum and put a 40 or 45 fused disconnect.
Be sure your main feeding cable is at least 10 or 16 sq.mm XLPE. If not, you must put a suitable main protection.
To protect the subs, use minimum 5*6 sq.mm...
