3/16" is pretty typical for bumpers, don't even think about using 1/4". For welding big pieces of 3/16" steel, 170 to 180 amps would be a good size for a 220v MIG welder to use. A smaller 110v 135-140 amp MIG welder would just be too small.
I would avoid the temptation to make a big heavy battering ram type bumper like some think is needed. The heavier your armor becomes, the worse your Jeep will do on the trail. A small compact bumper made with 3/16" is really all you need. I wheel in rocks and that's all I need, I seldom feel the urge to run into big rocks at speed.