B&B electronics makes a great ethernet to serial converter.... The ESP-901, you can do RS-232 and RS-485 to ethernet, I've used a few of them. You need to run a virtual comm port on the PC however.
If you are going to use a Lantronix device spend the few extra dollars and use the xpress-dr. There are some problems with the latest firmware in the MSS100 thats causeimg all sorts of problems. Not to mention the xpress is DIN rail mountable and can accept power from your control system.