Hi, I feel that your needs are different from the standard Sales Person. So it is not recommended to modify the standard function, a new function should be added. Perhaps the Item Attribute function can be substituted.
And on your line, there may be multiple Product Groups, so this field should be added on the lines if customized.
As for the development time, it depends on whether this field only needs to be on the order. If it needs to be on the posted page, Entries page, or report, you should increase the development time appropriately.
Hope this can give you some hints.
Thanks.
ZHU