if you're doing a GI Joe theme keep an eye at military surplus stores for some radio gear or other military electronics. It would be a much better starting point than a stock beige G3 case. A dull industrial box PC case might be a good starting point too.

Either way maybe add in a camo paint job. I don't know how skilled you are with some of the model making stuff, but deforming the plastic and adding in some other damage could be neat. If you take your idea and merge it with some of the game themed PC mods out there you could have something very nice.