The gearbox oil filler is on the front of the 'box. The drain plug is on the bottom, hidden by the gearbox mount for the dogbone. You'll need to remove the three bolts holding the bracket to get at the plug.
As for the hub nut - the Haynes manual for the 9N Polo says 50Nm plus a further 45 degrees for the 12-pointed nut type. Whether that holds good for the A2...…?