Beware of the cheap units like Harbor Freight stiff. They are ok for thin sheet metal but for frame work you need 220v. If you use an extension cord with the 120v units it is a problem waiting to happen. If the welder does not go up in smoke sooner than later the welds will be the one to suffer.. My preference is Miller, Lincoln, Hobart & name brands. pick up the name brand in one hand and the economy model in the other. The name brand is usually heavier, this is due to copper windings versus aluminum. The copper windings will always win out in the end.
INMO it's better to pay a little more up front and only purchase the machine only once.

Bob