You do not need to select the supplier – each stock item will have a preferred suppler applied to it and this supplier will auto fill when the PO is created

Each stock item/supplier combination can have its own price list set up (with price breaks if necessary)

The price list can be viewed in the PO Item detail page by clicking on the Price List button

You can now select the price that relates to the quantity required by double clicking – the selected price will then show in the Unit Price field of the PO item.