Stick your location in your profile
here, its near the bottom.
You can either go 1.6mm or 2mm wall with your steel. There is not much overall weight difference.
Good choice of welder, i've got the same one it should be enough to do the whole of the chassisand uprights ect.
I didn't feel that my welding was up to scratch so i ended up buying the wishbones and uprights.
Most importantly have fun, there will be some low points and unless you scavange or get really good bargins it will cost more than you think it will