Don't use Argon/co2 mix. That's for spray & pulse transfer mig. Using it with TIG will introduce impurities into the weld and will cross contaminate your tungsten into the weld also.
Helium is used for aluminum when you MUST have a hotter arc, but can't get a hotter welder, or increase your penetration without increasing your bead size & heat affected zone. It is expencive. Not only is it outright more expencive, but requires a significantly higher flow rate because of how much faster the helium disperses (even in overhead welding, tho it will build up in pockets once run). It is harder to light/maintain your arc, generally provides no benefit to the weld beyond 1/3 more heat input.
Stick with argon. With tig its a smooth running gas, good starting & cheap.
Helium has little benefit AFA applications that you will run into car wise. You will never find yourself needing helium.