Virtually anything can be done with enough work. I have and do make a lot of sheet metal panels and I can tell you that while it is not really easy it is possible. That said, it really all comes down to your skill level and tools available. If you make small sections at a time, thats the easiest way, only cut the piece that you are going to be making and use what you cut out as a template.
As to material, most people use either 18 or 20 gauge; 20 being the thinner but also the easier to work with.
One tip I will give you is this; where you see the rust is not the only place it is, meaning you will need to cut several inches around the rust at least to get to decent metal that you can weld to. The biggest issue is going to be blow through, where your welder blows right through the old metal. This is pretty common and something you will have to learn to work with. The way to do it is start your weld on the new metal and pull the puddle over onto the old metal quickly. In other words, let the puddle flow onto the old metal and it will melt and combine with it making the weld. If you are not going for originality, then I would suggest spot welding flanges onto the old panels, this is a strip of new metal probably 2" wide, half goes under the old metal and it spot welded through the old metal, then when you weld the new panel in it has something that it can lay on, plus it can be spot welded as well and then where the old and new meet it can be stitch welded (a series of small welds). This will give you a super strong floor plus the flange will absorb some of the heat making it easier to weld the old and new together. Its more work but you will get up with a better end result especially as it relates to strength. I would highly recommend that you do one or both of 2 things if you are going to try the flange method, first is to get some weld through primer and coat the flange where it will lay on the old and new metal, second is one done be sure to prime, paint and/or undercoat the flange/seam from underneath.
