I'd go with 304 stainless steel (A2 in the UK). 304L if you plan to weld it.
316 is typically used in a salt water/swimming pool environment so is a bit overkill.
Phisrace is right, stainless is definitely a bitch to work. I particulary enjoyed the fact that, with the larger diameter fasteners, you could be threading a nut down a bolt without any load on it, and the nut would suddenly cold weld to the bolt. How I laughed as I took out the hacksaw
Andy