The errors and discards may not prove to be as helpful as you think. This are measurements of bad and/or dropped packets in the inbound and outbound traffic on the NIC. What signs was that PC having when there were issues? Was there high CPU? Bandwidth from the NIC? You'd need to establish that before deciding what to monitor. I'd go ahead and add the workstation and monitor everything for traffic, cpu, and memory, and keep an eye on that machine. What changes were made on that machine prior to the network having issues? This would better determine what to monitor, or at least, what to focus on and setup an alert for.
Actually in the midst of a severe network outage where we could not see what the issue was, we asked ourselves - What changed recently? The answer was the addition of this machine the previous day.
We then noticed via SW that the switch that this unit was connected to was showing as down. We unplugged the new unit and bounced the switch at which point all of the network issues were resolved.
We need to add that PC back tot he network and wanted to monitor for trouble in advance of any cumulative problem which might again take the switch out.
Then I would start with just CPU, Memory, and Traffic and keep an eye on them to see if anything starts becoming an issue when the machine is added back.