Supply voltage will not make or break a good weld. As long as the welder will supply the amperage required to achieve good penetration, and supply enough wire to lay a good bead, it will make as good of a weld as you are capable of. If you have money to lay in the corner of your garage and do nothing in between your welding projects, by all means, get the higher dollar rig. If you don't, then get the smaller unit that will do what you NEED it to do.
I will agree with the others who have said PRACTICE. Get ANY scrap you can find - thick, thin, hold them together in different positions, and get to KNOW the welder you are going to use. Find out at what setting your welder will burn thru. Find out what pushing more/less wire will do. Find out everything about that welder and the weld that you can. Make a crappy weld? try to beat it apart. If it breaks, see where, and see what the weld looks like. Then, If you start a weld, you can figure out what you need to change to make it better.
So far I have NOT taken any formal training, have had a welder for just over a year, but have practiced a LOT, and can lay some very nice, penetrating welds. I have welded sheet metal thru 1/4" plate. I am still learning and getting the techniques down, but the results are good and strong. I have a 110v Lincoln wire feed with innershield fluxcore wire. Yes, I have to clean my welds afterward (shield slag - anyone who uses a stick welder has to do the same), but the welds are getting better every time I use it.
