Yes, you would need an Ethernet Module for the S7-300 PLC to enable connectivity to Citect. Otherwise you would require special comms card for the PC to work with Siemens Industrial protocols and physical network layer.
Another better option is to use the PSDIRECT driver for Citect. It is much cheaper and better performing option that S7NT driver. You don't need to buy Simatic Net software. But the PSDIRECT driver is licensed. See this site for further details: https://www.proscada.com/Citect.htm
Also contact your AVEVA Sales channel to discuss pricing on the driver.
