Purchase form with auto-sequential serial numbers

Hello

I have a excel based 'Purchase Order' form. Until now, my staff has been entering details on an open excel sheet which has too many problems such as over writing previously entered date, empty cells, etc

I was hoping to get some help writing a macro to create a form (attached file) with the below conditions.

1) The moment all fields in the attached forms are entered and the submit button is clicked, the data is moved to the purchase_listing and this form becomes blank and ready for the next entry.
2) Its mandatory for the person to fill all fields otherwise the submit button should give an error.
3) Lastly (perhaps the most difficult), P.O # should be an automated serial #. Hence when one form is submitted and the # is 213, the next form should automatically have # 214. However (here is the twist), if on a given date, even though 2 forms are filled at differing times but for the SAME SUPPLIER, the P.O # should be the same. The purchase_list has some draft examples.
4) I am assuming the macro will have no conflict if i have filters for some of the fields so that its drop down to choose from.

Thanks, your help is much appreciated.