I have these:
www.ti.com/product/LM331
but i never bothered to use them as the time is worth more than the price of THCAD.
Easy to use and can be plugged to parallel port with a single optocoupler.

Some clarifications before this goes sideways:
-COM0/COM1/COM2 are referred to serial ports, be it RS232 o other, usually.
-LPT0/LPT1/LPT2 and sometimes only P0/P1/P2 are referred to parallel ports, usually.
What are you referring to with COMn ?
Serial ports are pretty much useless with LinuxCNC, except for some stuff.
Parallel ports, also known as IEEE 1284, are used for machine control still, are perfectly usable with LinuxCNC, but have limited pulse rate capabilities compared to Mesa board.
Reasonable pulse rate or encoder pulse rate for parallel port would be 10000-20000 PPS, sometimes can go to 50000 PPS with a bit of luck/tweaking the BIOS.
At 3000 RPM, that is 50 RPS, so for a 100PPR encoder ends up 5000 PPS, and for a 1000PPR encoder it is already way to fast for most PC setups, see above.
Now convert that to mm/m, as you gave us no info about drive/gear/reduction.
After all that, yes, you can use a parallel port, yes it will work with encoders (2 max normally per port), and yes, you can lower the speed if you have higher count encoders.
But first, fix the wiring and get rid of interference/bad grounding.

alangibson wrote:

My intention with the TVS was to limit input voltage to 250V.

OK, but you must also add resistors in front of it, or it will blow up the first instance the voltage goes above 250V.
-
Personally, i have a strict policy of doing the voltage divider using 3 resistors, always, saves a lot of headache in case of mishaps or wires shorts or any of plasma wires touching ground or machine.
Example, in case of Hakan's schematics, i would use a 2K7 in the middle of two 50K. The other side of both 50K would go to plasma arc voltage, and the measuring would be done on the 2K7.