As far as the clothing, dressing up in vests, chaps, scarves, wrist cuffs, spurs, all that is optional in SASS or W3G. Blue jeans and a long sleeved work shirt, and you're good to go in either. No tennis shoes, t shirts or shorts, designer type jeans, not that hard to do. You don't have to be in an old west or hollywood western outfit.
Western 3 Gun still uses two revolvers, rifle and shotgun. They don't have a power factor requirement, but they use a lot of knockdown bonus targets that keep you from going to low in the power factor measurement.
A real old west cowboy probably didn't waste much ammo in shooting contests as far as wanting to be authentic. Of course to be authentic, you'll need to shoot blackpowder loads. I shoot blackpowder 44 russian and 44-40 loads. I have more fun shooting the bigger bores than I would shooting 32's or 38's. Mark