It is an M8 nut.
torque spec for that nut is 13 Nm
DIN coarse thread pitch standard for M8 is 1.25 mm. But I do not know whether that particular nut is coarse or one of the two fine options (1.0 and 0.75). I would guess coarse.

Any auto parts or hardware store should have that.

I definitely agree with the advice to make really really sure not to weld anything.

__________________
2001 X5 3.0i, 203k miles, AT, owned since 2014