Trained monkey looking to level up!

[m_c]

As it's parallel port, you probably want to have some kind of plan in place should the PC die.
Getting a replacement PC with parallel port is doable, but they are getting a bit rarer and generally only available second hand, plus you can't go any newer than Windows 7 32bit.

What you'd potentially replace it with is more a case of how much you'd want to rework the control cabinet.
Something like a CNC drive UC100, or PMDX-411 should be a pretty much direct replacement for a parallel port.
Or you could switch to LinuxCNC which would allow you to keep using a parallel port on newer hardware.

Off course, you could do a more major rework of the control cabinet.
It all mostly depends on if you feel you'd benefit from a more involved upgrade.