You can use the "Details" function and put the PO number in the "Reference Number" field. Use the "prompt for details on transactions" option (in Manager, under File -> Configuration -> Options -> POS Options). Unfortunately, with-out a third-party customization, there is no way to stricly enforce a reference number to be entered. You may wish to ask your reseller about this.
For a signature, you can also modify your receipts to have this line at the bottom - I've always put it in the footer section.
For lists of authorized people, the only way I can think of is to create a new "Shipping Address" for each person, then when you are processing the order you can select that person. You may want to use the "require customer selection for quotes/orders/sales" option, but I don't believe there is a way to prompt for shipping address selection without a customization. Again, you should talk to your reseller about these things.
Hope this helps.