If you want to weld aluminium properly you should buy an AC/DC tig welder but they aren't cheap, you can weld aluminium with some mig welders but you'll need a smooth liner for your lance and aluminium wire which is expensive and pure argon for gas.
Welding aluminium with mig is do-able but nowhere near as tidy a job as it is with tig and its easy to put too much heat into the metal so it takes a bit of practice.
All mig and tig welders need a shield gas to weld with (that gasless flux wire is cr*p don't bother with it), migs generally use argoshield gas which is a mixture of argon and CO2.
If I were you i'd buy a mig welder as they are easier to use, kempii minarc 180's are brilliant little welders for thinner metals but again aren't cheap, expect to pay around £450-600 for a second hand one, they're are okay from about 0.3mm steel upto about 6-8mm plate, they'll also weld aluminium with the above listed alterations, they are also semi auto setting if you desire so you can just adjust your plate thickness on the display and then weld without having to mess about with power and wire feed speed.
All migs will weld stainless, you just need to use stainless wire, you can weld stainless with standard mig wire but the weld will rust so its not recommended.
Stay clear of arc welders, they are for farmers and not suitable for automotive work.
Hope that helps