i would try to find one of these brands. miller,lincoln,hobart. stay away from the cheap machines and as mentioned stay away from flux core. you can weld sheet metal with it if you know what you are doing but its much harder to do and looks terrible. if you want the ability to do aluminum you need atleast a 220v machine and a spool gun. the aluminum wire has a hard time running through the liner and usually breaks inside.
for under 400 you will have to find a used one. if you want anything quality. most of the cheap ones will only run fluxcore. if you find a good lincoln or miller you can run both flux core and solid wire.

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