I think an air cooled torch would be just fine, watercooled ups the duty cycle (how many minutes you can weld per 10 minute period, IIRC. If the machine had a 40% duty cycle that would be 4 minutes welding out of every 10 at the rated amperage, the remaining 6 min. spent resting/cooling the machine/torch.) Like Zeus said, that shouldn't be a problem for your home shop.
Miller makes great welders, but don't rule out Lincoln...they will get the job done also. Gets to be a Ford/Chevy kind of thing...
A quick look on the Boise Cragslist turned this one up:
Miller Tig Welder
I am not familiar with this model specifically, but looks like it would work for you. It is a Squarewave machine, should have Hi-Freq for aluminum. I don't know what the box is on the lower portion of the cart though. Those big Argon tanks are pricey, I believe the last one I bought was upwards of $300. Being close by, you could bring over some sample tubing and have the owner demo some aluminum welds before purchasing.
***Please note that I have no experience with this specific model, have no idea if this is actually a good deal and by no means am I suggesting you purchase it on my recommendation. With some homework and patience on Craigslist though, you should be able to pick up a nice machine ready to plug-and-play at a good price.