nertajet hp125

Spent 20 minutes searching for it but could not find it.
I did an extencive reply on how to decide just that, so leme try it in a shorter version:
Does it have a divider or a jack ?
Can you find some resistors and wire them to make a voltage divider ? The values should be as PCW mentioned it i several posts, could not find them, my search fu is weak today !
If both are NO, then you need to get the 300 and wire it directly ( well also the above second version is wired directly ) , but need to find the right place to wire it.
Usually all decent plasma cutters have a filtering section that also protects the rectifier from high voltages during pilot arc, a smallish board with some resistors and some capacitors , use a DVM set to measure short circuits and put one probe of DVM to the plasma ground wire ( thick wire that clamps to the table ), with the other probe find the short to that board and connect the POSITIVE wire that will go to the THCAD through a resistor or without it. Do the same for the torch wire and connect the negative wire of the THCAD to the board.
To recap,
THCAD 10 if it has a divider or you want to make one yourself,
THCAD 300 if you do not want to make a divider.

It should also work with air, but the problem of the high cost of consumables remains.
You can also build your torch, just take care of the insulation, and do NOT use the same consumables as the price would stay the same beating the purpose.
You can change the cable and torch, might need some fittings made, but easily doable, cost of consumables would drop, but so will the cut quality.
The best idea in your case and the solution i always use for such cases is to add another cheaper plasma cutter to the system and have two configs for Linuxcnc that automatically switch between them at program start. This requires more wiring and a relay to switch the THC input, also separate relays for torch on signal.
This raises the initial cost, but the savings are really good.
And that gives you another discussion point when quoting prices giving them the choice of lower quality cut at a lower price and the high quality cut for a bit more price.
The torches i always use are Trafimet A141 or A161, the first with 12M cable set me back some 500 Euros, while the torchalone is 180 i think, i got several of them. The cost for a set of consumables is 5 Euro and they can do well over 1KM of cutting on a 3mm sheet. Yes that is 1000M, 1 Kilo Meter.