Does anyone know if you can set a price level for a Purchase Order and have all its items use it the same way as on a Sales Order/Estimate?

You should be able to set up vendor price levels although they are vendor specific rather than item specific like sales. They are also limitations around how the price is derived as you cannot use a "hard" price like sales, instead, only using a percentage off (discount method).

Ah that's good to know. I do need hard prices so I guess I will have to script it then