+1 to IAI, they are easy to configure, and work ethernet/IP to AB products.
As far as discrete control, that just means hard wired inputs/outputs, any plc with an input / output card (as long as its the same voltage type) will work.
I have never had to pay for their software so I believe it is indeed free or they just build it into the price of the component if not. The software comes inside the box with whatever product you buy.