My 2 cents, only get a C25 aka 75/25, for now with just learning you don't really want to delve into aluminum or stainless until you get mild steel down.

The only thing you can mix into the gas for aluminum is helium, it has some neat effects, can increase the power of the puddle by almost 50% but that's more for tig. Other than that, it's 100% argon.

Once you weld with gas you'll never want to touch flux core again. It's a very hot, dirty weld, the wire is a LOT more expensive to the point where the cost of the gas is negligible. The only thing flux core is really good for is when you need deep penetration or in a windy environment or perhaps in a repair environment where you can't get a welder and tank in easily.

I'm no expert in welding but I'm happy to help and share what I do know.