We recommend TIG welding with AC, 100% argon gas, and 2% Lanthanated tungsten. The most common filler metal is ER4043. For welds that need higher strength or will be heat treated, the ER5356 is the best. There are many types of AC TIG welders but we would recommend one of the newer inverter style machines because you can adjust the AC parameters and really dial in a beautiful weld. The Forney 220 AC/DC is a good choice.
Artistic TIG welding on aluminum will take some time to learn even if you TIG weld steel or stainless steel on a regular basis. Once you learn the TIG machine and the potential adjustments, the results can be spectacular. We always recommend using a foot pedal if possible as this helps you better deal with crater cracks by slowing down the amperage at the end of the weld and helps control the heat. Some common settings for aluminum is: balance at 60-70% EN, frequency of 60-90 Hz, 0.5 second pre-flow, and 4-8 seconds of post-flow. Some welders also like to add pulsing while working with aluminum.
Preheat on thicker section or large material is often needed because of the high thermal conductivity of aluminum.