welding fumes of any kind are very dangerous. That is why you are supposed to have a way to clear the fumes AND supply fresh air. Good shops and shipyards use a vacuum system close to the welding and a fresh air mask or helmet equipped with air supply when in "enclosed spaces".
Basically you will need straight argon gas and the correct aluminum wire. This means the correct type and the correct thickness. You may need whats called a teflon liner for your mig, since the aluminum is so soft, it likes to bend and break in the feeder if you have too much resistance in the line. The problem with mig'ing on smaller units is it likes to bead up and you dont get very good penetration. Aluminum needs lots of AMPS. Your even better buying a used 250amp tig then buying a new 150-180. I started doing a lot of aluminum welding so I bought a square wave 350amp tig and its completely awesome.
I've found the biggest problem with a MIG welder and aluminum is pushing the softer aluminum wire through the hose to your gun. We corrected that problem with a spool gun where the spool of aluminum wire is attached to the gun and does not travel through a hose. Yep, it's heavier but doable. Use 100% Argon with this setup. Ventilation is the key to happy welding.
26 years as a aircraft welder and 2 years in a production shop welding water vending machines for only 8 hours a day. Never have had one issue, argon is 1/3 heavier than air. That would means it runs to the floor not up your nose unless of course you are welding on the floor Your regulator should be set around 10-15 CFH (cubic feet per hour) a typical 2 car garage has 3600 cubic feet so if your garage was perfectly sealed it would take 240 hours to fill. If it make you fill better than do it, with all my experience this is the craziest thing I've heard. You have a better chance of getting metal fever or exposed to a hazardous vapor from welding than drowning from argon. The last part of your reply is why we have a confined space program (any space that could have limited air) My 2 cents
Here is some interesting information. I love the internet http://en.wikibooks.org/wiki/Wikijunior:The_Elements/Argon
Where I work, we've easily got 50+ guys welding constantly, 12-14 hours a day, along with 8 robotic welders going full blast. They can get a pretty nice haze floating around in there, especially up towards the ceiling. All the stinky ass Diesel tractors running around is what's really annoying, though.
