A $300 budget on a machine alone has limited your choice greatly.
To weld aluminum and stainless steel properly you need a very good setup. If you want a single machine to do both materials look into GMAW (MIG) or GTAW (TIG) solutions. With Miller, for example, you'd have to spend $1000+ US for their cheapest solution; either the MM 210 (GMAW) or Maxstar 150 STL (GTAW). Keep in mind thats for the machine alone, nothing else.
Good luck.
Seeing what you have to spend for a welding set up, you may want to invest in a welding course at your local community college first. Atleast you would get to use some welding equipment which welds what you are wanting to weld (aluminum, stainless,etc.) and talk to the instructor about what you really want to learn (to weld).
For welding stainless or carbon steel, GTAW or tig welding is the way to go and the set-up for that could be bought for about $600.00 if you can shop smart, but aluminum is a horse of another color where to use GTAW, you would need a high-frequency unit and an AC power supply which really starts to run into money. GMAW or mig is another alternative, but again you would need to invest alot more cash to really be able to do it.
It would seem wise to learn the processes to weld the alloys you want first and then save up the cash to buy what you really need. Even if you had the cash to buy both set-ups (to Tig & Mig) it wouldn't do much good if you have not learned properly how to weld.
Just a suggestion.
Be well,
Hawk
Well, i'm new in here, but i'm not new to welding, in answer to your question, If you're talking about aluminum, cast and stainless, then you could get by with a buzz box with a high frequency unit, which you will have to buy seperately, that will allow you to tig aluminum, but that depends also on how much tig welding you plan to do, i have used just such a set-up in the past and used a dry tig torch, and have had success with it, I will tell you that you will still be into it more than $300 though, the buzz box will take care of your stick welding for the most part, I would definitely get the a/c, d/c unit though, lincoln is pretty good. Lots of luck to you.