Ditto all Don said. I like flap wheels, both the flattish overplapping ones and the radial ones. I have a 5" Makita, and a Makita BIG electric die grinder for the corners and other tight spots.

I bought the Haynes welding manual, even though I hate Haynes, and it was worth it.

A couple of pairs of those welding magnets to hold your stuff while you tack it are really nice, and save having to hammer-square your work. I also got a lot of use out of c-clamp vise grips, both the wobble clamp and hard clamp type (and they work great for pushing the piston back in caliper on disc brakes), regular vise grips, and the like.

I say that the auto helmet is the best aid for the novice welder, because you can see exactly where the bead will start. The key is extremely steady hands, and a good view of the bead as you weld. The only problem I've had with the auto helmet is a tendency to darken when you look directly at a flourescent light. I'd really like one of the adjustable shade auto hats. Take a good look at the quality of the helmet tilt stuff, too. The hand shield in the kit is worthless, because you always need two hands, even if one is just steadying the other, unless you have the hand tremor control of a brain surgeon.