Use a MIG, and use a Argon CO2 mixture.
It's easier to get a good quality weld with Argon (or a mix) than with plain CO2.
Consider a reconditioned small industrial welder if they are available.
I started with 130amp SIP designed for the DIY market. It was Ok.
Now I use a reconditioned Migatrononic MX180. I got this for 50% of the price of a new one (300 pounds instead of 700).
It is much stronger and much easier to use than the DIY welders I had before and produces a much better weld. It also takes a proper gas bottle (about 4 feet tall) which is a much cheaper way of buying the gas and full size wire reels (5kg or 15kg instead of 0.7kg) which is a much cheaper way to buy the wire.
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