For a featherweight look at 10mm by 10mm or 15mm by 15mm and a mm or two thick steel box section. You can roughly calculate what weight your chassis will be by working out the weight per mm length of box section and then total it up based on how much box section you would use. For a supplier best to look locally. Look in the yellow pages for a steel or metal supplier.

and with regards to gearing your axe down, calculate the gearing that you need and then we can better advise on what would be the best method for gearing.