Howdy,
Much like a Porsche, Tig is a highly refined welding process.
There are a few different size dinse connectors industry wide. I would surmise that your new 250EXT will have a .505 dinse connector 1/2".
Collets and collet bodies are standardized for torch sizes. Again I would surmise that you will be getting a WP18 torch with your machine. These will fit other brands of the same size torch. Gas lenses as well as cups also are size oriented.
Water is still the same everywhere unless it has a color (just kidding). Usually the interfaces are standardized.
As much as we would all wish, the foot pedal isn't stardardized. Usually by brand and machine.
And you are correct, gas feed is pretty standard.
Once you get the hang of it, Tig welding is pretty cool. But learning can sometimes be like changing spark plugs on a 911 without the right tools. It's all worth it!
Have fun with your new machine.
Many of us who have Everlast Tig welders have upgraded to a WP20 torch. The WP18 is good, but the WP20 is smaller and easier to handle. The great folks here at Everlast can help you out with it.
Everlast 250EX with cooler and WP20 Torch
Millermatic Mig Welder
Gas welding setup
A bunch of Snap-On tools
And a Brain