I agree with Welder Guy. .035 wire is extremely versitile, and economical. Although .045 will produce higher deposit rates, it's a bit much to handle and takes all the fun out of it. I think .030 is used mainly on sheet metal. Go with ER70S6 wire. (Although I'm not sure you could tell the difference between 6 and 3). The problem with GMAW low voltage (short circuit) all position welding (especially on the metal thicknesses that your working with) is lack of fusion along the sides of the weld. Don't try to carry too much metal. In many situations, your better off running 2 or 3 passes instead of trying to make the weld in one pass. The travel speed is relative to the voltage and amperage settings. More wire and heat, faster travel. Less wire and heat, slower travel. Got to say it one more time, always keep the wire squirting on the leading edge of the puddle, that's the important thing.