Although .120 wall tube can be done with 120v welder, it is at the top end of it's capability for an experienced fabricator. Step up and get the 220v, it will be money well spent.
As far as welding up a cage, it is one of the most difficult projects an amateur can do. Welding a cage properly starts with proper fit up, that means notching with NO gaps. You are not welding horizontally on a table, you will be welding out of position, up, down, sometimes upside down. Also, since you are welding round tube, your torch angle is constantly changing as you go around the joint. You also transition from a lap weld to a tee weld as you go around. This obviously should not be your first welding project.
One common mistake I have seen is when using hole saw notchers is not properly finish grinding the cope. Some people leave the "flash" on the end of the tube and try to weld the thin material left at the longest points of the notch instead of grinding it back to the full thickness of the tube. This makes for a scary joint.
A great way to practice and test your welding skill is to buy some 1.5" x .25" strap steel and cut it into 4" pieces. Overlap two pieces and do a lap weld across one side. Put it in a vice, grab a hammer, and try to break them apart. If the weld fails, you need to keep practicing. If the material fails, the weld is good. You will find that even some of the prettiest welds will fail. Beginners tend to use too low of a setting, causing poor penetration, even if the weld is pretty looking. I once broke a front spring hanger off of a SAS'ed Toyota with a pry bar that had been welded by a "fabricator". The welds were pretty, but had no penetration. The customer happened to be standing there. Needless to say his jaw hit the floor.
All of you who are saying how easy it is to weld a cage, especially those using 120v welders, try this little exercise. take two pieces of tube, do a 90* notch on one piece, then weld a tee shape 1/4 of the way around, half lap, half tee. put it in a vice, and break it apart. What failed, the tube or the weld?
Post up your results!
P.S. One thing I do not like about the Metal Tech cage is the small brace from the main hoop to the halo. It is too close to the occupants heads, and could cause a serious head injury in a hard roll or even a violent accident. I would make it much smaller, or just use a triangular gusset in it's place.