Okay, glad that you're already aware that everything about this is a bad idea, except that it will definitely look cool... so with that being stated clearly:
If you were going to go with any armor maker to do this, AM is the way to go, as they are one of the thicker and stronger kits available. Since they utilize metal molds and industrial machines when pulling their plastic, they can go a little thicker than most and still capture excellent detail. Definitely more accurate than a Rubies kit.
As far as damage to the suit, if you do anything other than stand around and take photos in it, you're probably going to damage the armor. The original armor was literally thrown in the trash after stunt filming, so usually we take great care NOT to be doing the exact type of running/dodging/etc typically found in airsoft or stunt work. Be aware it is very difficult to lean, kneel, or crouch, and sitting is usually off limits as well except under certain conditions. The 'correct' chelsea boots are also not terribly comfortable and don't have good grip for running. Heck, if someone bumps into you hard enough or you fall down, you're going to have a bad time. Repairs are possible for small cracks, but a large crack could destroy a piece. I can't see this being a regular thing, but it'd be a fun $1500 gimmick for a few rounds.
You'll undoubtedly want to make some mods to increase mobility (almost more akin to those Boogie Storm clowns), but I'm sure the kit could be cut to some type of compromise between some level of comfort and still looking like a stormtrooper. As far as the impact resistance... There ARE thinner spots that might not take a direct hit from close range, but in general, BBs should bounce off and be fine with some scuffs or dents at close range (there ARE still several gaps in the suit under which a thin compression suit is typically worn, so that might still hurt). The BBs aren't going to be your problem, it's going to be the terrain you're interacting with and your mobility/vision.
EVA foam could be a good alternative for comfort, but wouldn't look as good. That's been done before with interesting results...
So idk, there are some thoughts on that and I guess an endorsement for Dave, but yea... I would never invest the money to do this, but would love to see you do it if you're dead set! Post pics/video hahaha