Using a DC welder is way too difficult with any gas, if the wrong polarity is used it will ball the tungsten and melt down. If you are on a close budget, try to find an old AC welder like a Lincoln 200 or 250. An air cooled TIG torch can be connected but it will be a scratch start. Use 100% Argon, helium costs too much today. It is not used much in the Industry today. Sometimes I will use an Argon, Helium mix depending on what I am working on and 100% tungsten to learn since this is old school.
Aluminum needs to be clean and warm. Wash with acetone only. Other solvents are oil base and will leave a residue. Use a soft bristled stainless steel brush to clean the weld area and wash. Warm with a propane torch til no more moisture occurs. Try making a puddle and practice manipulating the torch for a while before adding wire. Any aluminum can be welded, even cast. Check out TIG Time on YouTube, this guy knows his stuff. Good luck. Doof