RE: Deleting Old Purchase Orders
Ideal way is to Invoice this PO and then reverse the entries (by Credit Memo or Journals), if you have already paid the amount to Vendor. This would ensure, that you do not process the duplicate payment.
If you do not do this, you will have interim amounts in the G/L Ledger and also your inventory valuation would be on toss. So best way is to always Invoice the old receipts which are not invoiced.
Going forward, you please setup the correct procedure to close these entries on periodic basis (like Received not Invoiced, Shipped not Invoiced, Transfer Shipped but not received etc.)
Otherwise, you would have Incorrect Inventory Valuation at the same time incorrect G/L Ledgers.
Alternatively, you need to work on lot of Interim Item Ledger / Value Entries (Expected Cost) & G/L Interim entries. This would be very tedious and inappropriate task.
Hope this helps.