Lets look @ the options:
Ally: light, easy to work, crap under stress, not easy (or cheap) to weld. If you really must use ally, go for duralumin, its the same stuff they construct airframes and aircraft skins from. Its an ally alloy. the most important thing to consider is where you expext to use it......
If ever you intend to go anywhere the sea / beach, make sure its properly painted, or it will corrode away to nothing. salt water and ally dont mix well (that includes if you transport on a roofrack, salt from the roads in the winter will bugger it in no time.....)
If you are still intent on using alluminium, you'll find it under something like 'non ferrous metal supplies' in your yellow pages.
Stainless steel, dosen't rust right? yes and no, cheap stainless rusts, its just not as bad as mild steel. if you want true stainless, you need a good grade (think chemical industry grade). If you intend to drill it, make sure you got some drill bits capable of drilling stainless, or you'll burn them out in no time. Drilling needs to be slow, and use a lubricant to aid cutting (it helps keep the tip cool, 3in1 oil will do) If you intend to weld to the stainless, it will really need to be tig welded its all down to how many bucks you want to spend. If you have the cash, anything is possible, its just if you are working on ideas / prototypes, its gonna be expensive just to test an idea.
There is a third option, its cheap, widely available, easy to machine, work or weld....
Mild steel. it'll do anything you want of it, if you find your idea needs modifying, then you can start again till you get it right. and dont worry abour rust, a good coat of paint or grease will mean you can buggy in the sea all day without any worries...
think of it this way, if its good enought for a Vmax, is it good enough for your buggy????
hope this helps