Steel

[baz-r]

go for a pre cut if your building your own (unless you work in a fabrication shop)
it saves so much time and trouble and you will be able to progress quickly
i would go for a handyandys "2mm" or 3ge(john) "1.6" both are on this forum and will be able to help if you have any probs and thats worth more than a few quid
plus you have to look at mistakes you might make and the cost of cutting discs saw blades and so on

also what do you plan to do with the chassis? racing and bike engines 1.6mm light.

big bhp and v6 motors go 2mm