Some shop notes on TIG torches:
175 amps is my limit for comfort with an air cooled torch. The most common size air cooled torch out there is probably a 17 series and they are rated to 150 amps. They can be used intermittently above 150 amps but it isn't recommended and it won't be comfortable for long. I think the biggest air cooled torch currently made is a 300 amp model made by Weldcraft. Don't ask me whytf you'd want to use air cooled torch at 300 amps, but it's possible if that's the kind of thing you want to do.
The most common water cooled torch you'll see in non-specialized repair shops is a 20 series. They are rated to 250 amps by most mfg's.
Most manufacturers use the same model numbers for torch sizes/types, so it's semi-standardized. The actual model numbers don't really follow any logical pattern, I think it's some system Linde/Heliarc originally came up with back in the 50's and other manufacturers just went along with it. An "F" somewhere in the model number usually means it's flex head torch. Here's a link for CK if you want to get an idea what the different style torches and front-end parts look like.
LINK to CK At a minimum, know 17,18, and 26 series torches all use the big type collets, and 9 & 20 series use the small type collets.
An air cooled torch is nice if you're doing a small job out of the back of your truck somewhere because it's means there's one less machine you have to find an electrical outlet for. Even if the welder never leaves your shop, it's still good to have an air cooled torch for backup to allow you to perform small welding jobs if there's ever a problem with your water cooler. As mentioned above - do not run a water cooled torch dry! It only takes a minute to connect an air-cooled torch if you can't use your cooler. Connecting one dinse plug and a gas fitting is usually all there is to it. There's nothing to "convert" on the welder itself to revert back to air cooled. If you are using a finger amptrol on the torch, you'll have to swap that over too, but 90% of the people out there are using a foot pedal amptrol. You can leave the cooling lines on the water cooled torch connected.
You will probably want to have at least 2 different torches whatever style you decide on. Making small precision welds with a huge torch is possible but it can be awkward. You want to match the torch you use to what you do most frequently.
Used TIG torches can be hard to sell because there's such a small market for them. If you know exactly what you're looking for, it's a buyers market. It's rare to see a *used* air-cooled torch go for more than $75 on Fleabay. If it's missing parts or incomplete they can be had for under $20. You just have to know what parts will work and what parts won't.
If you plan on doing aluminum be warned: It doesn't just get hot at the weld - it gets hot all over!!!! I use a small reflector type heat shield for AL work to help keep my torch hand cool from the added reflected/radiated heat you get when doing AL. The hand heat shield costs something like $7.99 from my local welding supply and it's saved me a lot of crispy fingers.
Once you get familiarized with what effect all the different variables have in TIG, it isn't as mysterious as most people make it out to be. It is an expensive initial investment and the learning curve is steep at first. If you can gather up any AC/DC TIG with a remote amptrol, Hi-Freq, water cooled torch, and cooler for under $1000 I'd say it's a bargain.