Hey edvwboston,
Since I have 2 of the older Chicago Electric MIG welders, I can say the work very well and were well made. Mine were made in Italy, and it sounds like you have one that was made there also. If it's a 120v model, the output range is probably 75-100A. That is good enough for sheet metal and up to 1/4" with some practice. A bottle of gas, depending on size can range from 80.00-300.00 unfilled. A fill, depending on size, ranges from 26.00-140.00. The regulators, even used, are in the 40-80.00 range. The price of 200.00 sounds quite reasonable, but I would have him run a test to show you that it is working properly, and weld a piece of 1/8" just to be sure of its capabilities. A 110v unit will not be as efficient or have the heat range as a 220v unit, but will weld nicely on lighter material. There's no "heating up" with MIG. It is an instantaneous weldment created with an electric current arc. I can't say if parts are available, but I did find a gas solenoid for my 160A unit from the solenoid manufacturer who originally furnished them to Chicago Elec.. Sometimes you can find parts easily on E-Bay.
If you are inexperienced with MIG welding, do yourself a favor and take a class at a high school or local college and by all means....PRACTICE!!! Hope the helped...ask any questions you are unsure of....Denny