Welder advice - where to start

I don't know about all of the welders but for what you are asking, I know this will be perfect for you. https://www.lincolnelectric.com/en-us/Equipment/Pages/product.aspx?product=K2471-2(LincolnElectric)
The C in the model designation stands for constant. No detents in the heat setting gives you ultimate control for those perfectly deep penetrating flat weld. Practice,practice,practice. Buying from a dealer is a better machine and a little more money than the box stores.
And as already mentioned, 75/25 with argon is much better than straight CO2.