OA has its place, it will weld and braze but with the advet popularity of wire feeders it isnt use as much for welding anymore. It is part of any shop though, for cutting and heating. I would be totally lost without one. I agree with the Miller or Lincoln choices and there are a lot of them. I am not a big fan of 120V units, the small 230V in the 175 class will do most things in a car shop on steel. For exotic metals
TIG is used and I am not as familiar with all of the models, I would likely go with Thermal Arc, TA185, probably around 1500 but its substancially less price of the next similar setup and works very well. You can use a cheap wrench or hammer but the technology that drives these machines is different and in order to make cheap machines they cut the things that the major players use to make them to work well. There is more than price at a certain level,,, its how well do they work, that comes with a certain price.