I'll be cheeky after a glass of wine on a Friday Night!
Throw out your current hardware. Buy a Mesa 7i76e ethernet card and one of their THCAD-10 boards which will cost you all of USD $268 all up, far cheaper than most THC's. You can buy the gear here and the manufacturer's site is here.
Install the LinuxCNC plasmac config. I've written an installation guide here. Its actually got even easier as there has been a configurator written in the last few days.
There is not one external THC that can hold a candle to this plasma controller. The reason for this is that the THC function is fully integrated into the motion controller so a simple M3 starts the whole probing and torch ignition process. Jim Colt from Hypertherm says that the best THC's are fully integrated into the motion controller. He was really referring to Hypertherm's high end controllers never imagining that the Opensource movement would create an equivalent (some would even say superior) system.
Here is a video today
And another one showing how well it manages torch height control with slopes higher than what you will get when cutting normally. Just before I made this video, I had been playing around with probing speeds so the slow probing is user configurable as I had slowed it down to accurately test the probing height.