Those welders would work for getting a brew stand built provided you make it out of steel. For the smaller lowr amperage models you have to keep your steel at 1/8 thickness. For example: if you are making your stand out of 2" sq. tube make sure it has no more than 1/8" wall thickness (plenty strong). With the larger machines you will be able to weld thicker stuff, maybe up to a half inch.
I cant really vogue for the quality of these machines, I know others that use them without much problem but never have for my self.
At Harbor Freight it can be kind of a crap shoot.
The only problem with the mig is going to be welding SS. You will no doubt get a very ugly weld that is contaminated and crystallized. Their is an advanced technique that is called "drip transfer" it involves welding SS at high amps (temps) and lower wire speed with the mig. The idea is that the metal will drip ot kinda spray. Wit a steady hand and a little "c" motion in the drag and you will get a nice looking ss weld. Not as good as TIG but nice.
As you may know, you can put just about any ss weld on your kettles cause sanitary isnt really an issue. But welding SS on fermentation equipment is very tough. Welds have to be damn near perfect.