HI,
Regarding customer master making tax classification field as mandatory, you can do that using customer account group in obd2 where choose the account group that you are using and make tax classification field as mandatory over there.
Regarding putting Tax field in sales order header level incompleteness log goto ova2 --> sales header --> standard order ---> add tax field in the fields tab then it will be included in incompleteness log.
Regards,
samba
