Hello!
I would also add:
3 check the gear tooth surfaces for wear.
The lower row of the gearbox may not be concerned because it continuously having oil.
The upper row must be checked, especially the gears from the upper row to the vertical head (the last gear close the the front horizontal bearing spindle - and the one of the horizontal shaft gear) This parts might be more worn that the rest.
I would also suggest to make a short try without the complete vertical milling head (as you would do an horizonzal milling)
This just to be shure the noise come effectively from the gear box.
As Doctor_itchy wrote, please check all before full use
See you
Jim