Weld it on as flat a surface as you can.

If I was going to do something like that, I'd be buying a big spirit level to ensure the main frame was set as level as possible to welding, and a big set square for ease of checking things are square during initial setup (I'd also be measuring corner to corner to check squareness after the initial tacks are in place).

If you order metal pre-cut, you may still have to grind it to length, as cut tolerances can be quite big (typically -0mm to + several mm).

As for welder, I'd go for something in the 150-200A range. You won't need that much capacity, but at that rating you shouldn't need to worry about duty cycle. Ideally gas MIG is better, but the cost of gas can mount up if you're using disposable bottles, and the rent on full size bottles is quite costly. There are suppliers where you can buy bottles, but you really need to find one locally, and check to see how much the gas works out to per litre (some of the schemes where you buy bottles don't fill bottles to that high a pressure, so you don't actually get much value for money).
Other option is gasless MIG using flux cored wire, but then you have to deal with flux/slag cleanup afterwards (best way to describe is, is it's pretty much ARC welding, but with a roll of wire instead of rods).