Bigger MIG welders are rated to a standard- eg 200A @ 23V, 60% duty cycle. The cycle time is out of 10 mins I think.
REAL big Amarican ones are rated to 100% duty cycle, eg 600A @100% duty cycle.
Some of these tiny ones seem to be rated at what they can give for a fraction of a second. So beware.
You don't need a big machine for car panel work, even 110A is plenty- but you want good voltage control and arc stability.
These days I won't buy without trying it first.
Gas or gasless? I prefer Ar/CO2 gas and solid wire, if the machine has a gas solenoid it can run either type. You have to hire the gas bottle, unfortunately. Don't look at the disposable gas it lasts about 5 mins.
Gasless are cheaper machines to buy, but the wire is lots dearer. And the process gives off lots of nasty fumes.
IMO the only advantage of gasless wire is for welding galvanised steel or outside in a breeze.
<edit> Mine is a Kemppi `Super Kempomat' 125A, has stepless voltage control. I bought it s/h years ago for $450. New they were around $2K once.
<edit 2> Beware of the cheapo Telwin gasless, it has NO wire speed control and only 2 heat settings.. a waste of time, IMO. Only current adjustment seems to be the arc length you hold...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
