The first thing you have to do is get a better handle on your expected load. You do not to build this to run everything if you are only running them one at a time,. Then go 125% of the biggest load.
There are also rules that allow smaller conductors for welders but you probably should get some engineering advice that may be beyond the score of this group.
You are right about separating the neutral and ground. You also do need 4 wire cord.
