Hi Paul

The easiest way to do all of the above would be to create a custom field (or fields) on your records. You could simply create a checkbox on an Invoice record and one on a Supplier Order record which you can tick when you've been paid/made payment respectively. This would allow you to create a filter to show any invoices that haven't been paid.

If you need more data, you can take things further by creating a couple of custom fields: one to show the amount paid and the other to show the date. This would enable you to build a report showing how much was outstanding (by creating a formula that takes the amount paid from the total amount due) as well as being able to calculate things like how long on average a particular customer takes to pay.