As said above, TIG welding is the hardest of the skills to learn..
I sell all kinds of welders on a daily basis, so had to learn every type of welding process.
Cost isn't an issue, if you want to weld mild & stainless, as a reasonably basic MMA (ARC) welder will do.. just add a valved (WP9) TIG torch to it, and off you go.
However, you won't have the joys of footpedal control, or HF startup. MMA sets use the 'scratch start' , or 'lift arc' process. Basically use either scratch the tungsten on the material, or touch down and lift up slightly to initiate the welding arc.
For Aluminium welding, you need an AC/DC set.... much more expensive, but once you get into it, is a great bit of kit to have... plus can make you a bit of cash on the side... There's always people out there wanting something fixed...lol.
Gas is definately the key element here too. Argon/Co2 gas that you usually use on MIG won't work very well. Best to use pure Argon, although there are other, more speciallised mixes out there which work even better.