Welding

I'm after some advice but haven't found an exact answer so I'm hoping someone else can help me out,
I've got to weld the outer corners of the footwells and I'm wondering if the steel I have would be any good? It's the side casing from an old boiler and measures about 0.5mm although it still as a layer of paint either side, would this be any good? If not what is the size I would need? Also I have a very tiny hole near the front spring top on the chassis, (about 1cm long x 0.5cm wide) would it be worth putting a plate on that or just welding it up?
Is 1mm steel ok for the chassis?
I'm trying to get all the welding sorted out so I can get it all painted and waxoiled before it gets bad.
Hope someone can help
Cheers
Dan

1mm for the footwells 1.5 or better 2 mm for the chassis and plate over the hole,

Cheers bud much appreciated
Dan

The hole at the front outside face of the chassis is very common, I welded two identical plates one each side out of 3 mm steel neatish and looks quite smart, also the steel around the hole is normally pretty sound, unlike the rear crossmember!
2mm steel for the chassis patches is a bit thin, certainly wont last.