Buy the floor panel, it's much easier fitting than stuff not pre-bent. Check my b-II build thread for what I did to fix rust. Still driving it after three years and it still looks like crap but I'm not worried about falling through the floor. That welder would do just fine with practice. You just have to make sure that you are welding good metal, otherwise lots of blow holes. The trick is not to weld it continuously but to use a tac-tac-tac method and move around the clock to let the metal cool between tacking ~ 1" at a time. Another option; I also used self tapping screws in a lot of places and they are holding up just fine. If you have air, a flanging tool would also help in the fitment for welding, it creates a 1/2" lap that the new metal falls into and makes it a lot neater looking.
Good luck,
Richard