In addition to what's already been said, a machine that allows you to adjust the AC dwell in Pos. and Neg. will be a bonus. When the current flow into the electrode it has a cleaning action on the work and when it flows from the electrode it penetrates more. Depending on what you're welding, new 1/4" plate or a nasty, broken Sprint Car Cylinder Head, That adjustment can really pay off. With new metal you can adjust for mostly penetration which will also reduce the width of the 'abraded' area next to the bead so a fresh weld will be very cosmetic.
Also, generally a green tungsten (pure tungsten) is used on middle thickness (~1/8" - ~1") aluminum. When you get above or below that approximate range, or you're doing something special, then you might use a red tungsten (2% Thoriated). It's not that you can't weld aluminum with AC and a red tungsten, it's just easier to start out with a green. Wait until you're fairly proficient with ferrous metals b4 trying aluminum. I've met one person who started on aluminum and had an easy time learning to weld it, but had trouble with steel. Most people are the other way around. My general philosphy with aluminum is "Stuff rod in and GO !!" You can't hang around in one place very long like you can with steel.
Old school has you striking a reverse polarity arc on something like a slab of copper with a green electrode to ball the tip prior to setting the machine AC and welding. I don't bother. The electrode will ball up reasonably fast. I just grind the end square and start welding. If it balls up too big and threatens to, or does drop into your puddle, you have too small of an electrode diameter for the current.
Red electrodes are commonly used on ferrous metals. On the thin stuff in particular you want to try to match the tungsten diameter to the metal thickness.
I wouldn't bother with R. Finch's "Performance Welding". I met the man and he knows a lot about welding (& about Corvairs too), unfortunately it doesn't come out in his book. If you want a book, find a real text book on the subject.
FWIW I find that stainless is the easiest to teach people on. Mild boils too easy and with the concentrated power of a TIG that is easy to do. Stainless is not only more forgiving of that, but it also wets out and flows easy and rewards with pretty beads fairly early. A little ego boost about then is a good thing.
Have any of the other TIG welders out there fiddled with the Yellow or Brown electrodes ? What are their uses/applications ?
TS
"Teach you all I know and you're still stupid"
-- Howdy Lee