3. as far as battery choice is considered, internal resistance is most important stuff. Too high, you wont be able to weld well, too low, and you will blow mosfets. This is a simple circuit, and only thing limiting current is resistance in battery and connections. So, in particular, I recommend 45 Ah to 90 Ah battery for standard version (380 to 760 cranking amps) and up to 130 Ah battery for stronger version. I haven't tested other types of batteries, such as gel, AGM, or various li-ion batteries, so I can't tell anything from experience, but you can calculate their internal resistance and match it to starter batteries. And you can experiment of course. I have named what works well, so that's a good starting point, and honestly I don't see a need for using anything else because starter batteries are great for this purpose, and they are cheap any widely available.