you start with a simple AC stick welder, add on a diode box and then you got AC/DC, add on a spoolgun/readywelder and you now got fluxcore MIG, or add a bottle to that and you got MIG. somewhere allong all these addons you could also stick in TIG. adding a carbon arc torch doesnt even need mentioning, does it? :)
so you get an AC stickwelder, and you can end up with MIG/TIG
probably for a lot more than getting separate welders, and probably with poorer results, but possible, right?