How do you set up early repayment discounts for purchases rather than sales? Also, just general discounts for purchasing stock?

purchasing stock as inventory type items, you use the item details part and select the item, the qty purchased and the actual cost - you do not enter sales qty discounts separately

early payment on a bill from your supplier, create a supplier credit memo, use an item that post to other income named something like early payment income.  use that item on  the credit memo, then in pay bills, apply the credit memo and pay the balance