The first decision with TIG is DC or AC/DC. DC is enough for mild steel, stainless steel, copper and titanium. Aluminium and magnesium need alternating current, so if aluminium is in your plans, take AC/DC. Adding that function later means replacing the machine.
Match the current to the thickness: 160 A is enough for sheet up to 4 mm, 200 A up to about 6 mm, 300 A and more for thicker joints and continuous work. With TIG the minimum current matters just as much. A good machine goes down to 5 A or lower, which lets you weld 0.8 mm sheet without burning through.
Look at the ignition. HF is convenient, but in a workshop full of electronics it can cause interference. Lift arc ignition by contact is quieter, though less precise on thin stainless steel.
Pulse helps on thin sheet and when welding out of position. On AC/DC machines check the AC frequency range, usually 20 to 250 Hz, and the balance, usually 20 to 80 percent. A higher frequency narrows the arc, a larger negative portion gives deeper penetration.
For work above 200 A on long welds a water-cooled torch is worth having. Check in the listing whether the cooler is included or has to be bought separately. We always state that openly, along with information on the foot pedal and the cables.