There is an unanswered question from the original post - has anybody used a LapLink cable over the parallel ports for TCP/IP networking? I suspect that there is a packet driver out there that does PPP over the parallel port, but I've not tried it myself. And it would require you to have a gateway machine that could act as a router to feed the DOS machine the intended TCP/IP packets.
I've done this for PPP and SLIP over a serial port using a Linux machine as the gateway. But not using the parallel port. Has anybody else done this? How about using Windows on both machines?
The Xircom is an extra cost option. But it does get you out of the requirement for a gateway machine. (The Xircom PE3 connects directly to a modern switch or hub if you have the right version. Some versions use ThinNet, so unless you are familiar with ThinNet avoid those.)