Can anyone give me an ideal of which circuits should be fused, amperage of the fuses, where the fuses should be placed, etc on a BN6?

Depends on how many fuses you want to add.
First would be the red wire on the back of the lighting switch. This wire feeds the front & rear running/park lights & most importantly, the number plate light. If the car gets the rear bumper whacked it frequently cuts into the number plate light feed & causes a short circuit. 10 amp USA inline should work.
An alternate would be an inline fuse in the boot just for the number plate light.

Next would be the white/green wire from the OD relay to the gearbox 3-4 switch. If the contact on top of the OD solenoid fails to open as it sometimes does, the OD solenoid will rapidly burn up. 20 amp USA should work.

Sometimes folks fuse the fuel pump supply lead, one of several white wires at the fuse block. This wire is kind of vulnerable to getting mashed under the frame. 10 amp USA would be ok.

These last two are readily acessible at the existing fuse block & you could add another OEM fuse block next to or below the existing block.

This is only my suggestion. Other folks may have different ideas. Any additional fuses are better than nothing.
D