I would buy the biggest capacity cylinder you can handle. The gas is cheaper per cubic ft and you don't have to swap cylinders as often.
Around here a cylinder 330 ft cylinder costs around $300 for the cylinder and I pay $35 or so for the argon.
DAMN... its like 110ish here for a 330...
all the chain gas supply houses want a lease on that size cylinder.. there is only one place I can take mine since I own it..
OP, listen to these other guys wisely...
I bought my Tig strictly for aluminum and chromoly cages.
I weld a lot of ****** cast alum Cobra and Shelby blowers and intake manifolds.
I have 4043 for clean alum sheet but most use 5356 on the dirty cast. it doesn't lay as nice and pretty as 4043 on clean sheet.
Get a stubby gas lense kit. when welding alum, especially used and dirty stuff cleaning is important. grind and scuff, then acetone the parts, and the filler rods.
welding clean sheet aluminum is easy peasy. start off with mild steel first in my opinion. i use 3/32 tungsten for almost everything except super thin sheet. I use 1/16 filler rod in mild and SS, then 3/32 in aluminum, but most of my alum is very thick, requires extra heat to flow the puddle properly. 1/16 would be better filler for thinner alum.
70s is all I used for mild steel. 70s also works for welding SS to mild steel. so does a normal mig gun...
I have 308,309,312 and 316 rods. 308 is going to be the most common used. 312 is great for welding two dis similar SS, or SS to mild steel.
312 is also great for ductile iron/malleable iron repairs. it has a lot more Chromium in it that helps from cracking on the minimal carbon content irons.
there are a lot of welding snobs that will tell you, you have to do this or use this rod or blah... end of the day... finger time is what you need. its not hard. you're welds will look like fried dog **** for awhile. most times when starting on Tig guys use to much heat. Weld.com has a bunch of techincal charts and data to help out beginners. they also have quite a few videos on youtube. but nothing beats hands on help, if that makes sense.
Good luck