Hardware questions..

[mtdriver]

You do want M3, if you pick that size on McMaster it should default to the common pitch (maybe thats all they sell, dont remember). It is more commmon to see a thread diameter by lengh listed on metric than thread diameter by pitch like American hardware. Also go with stainless, it isnt quite as strong, but will not rust and look good.

3x8,10,12 and 16mm are good sizes to start with, maybe some 20mm if youd like. Get some washers and locknuts too, standard nuts if you will weld/braze them to anything.

Have fun!