I own a welding supply business. 140A is OK for sheetmetal, anything much bigger will not penetrate and you will get something that looks like a weld but is not.
180A is a nice size. We sell both Lincoln and Miller, they are both fine. A new option is Forney. The Forney uses standard Tweco consumables and has the heavy duty wire drive like the industrial grade Lincoln machines.
I personally would not buy a Harbor Freight. If it needs repair you are SOL.