Tanks are rather expensive due to the cost of the steel (which is what you pay for) for a 125 Cu ft bottle your looking at 200.00+fill. To give you an idea of how big 125cuft is, 4' high and 6-7" round. You dont want a tiny tank otherwise you will be filling it all the time.

As far as gas you want a 75/25 argon/co2 mix. You can run 100% co2 like I do but it increases your penetration and makes welding thin gauge a little tricky. You can also use 100% argon. For gas you need to consider the gauge steel you will be welding and what gauge wire you will be using. If you just want something universal then go 75/25 and a .030 cu coated steel wire.

Terry