Bookhound,
You can use just about any gear you want, but just remember this is and IPSC style match and not a tactical match. There won't be any tac gear/vests, thigh holsters, etc.. Lets start with rifle. Most shooters will have some variation of the AR. There are two basic classes you can shoot in, Open or Limited. Basically for rifle, the only difference is optics and bipods. If you use a bipod or optics, any optics, you're in Open. If you shoot iron, you're limited. Beware that if you shoot open in any catagory (rifle,pistol, shotgun) then you will be open across the board regardless of what you shoot. Shotguns are pretty easy. For limited, no optics and 9 rd maximun. If you use optics or load over 9 rds. your open. For pistol, its pretty much the same. Scopes and comps and your open. Irons and your limited.
As for the actual match. The rifle stages were typically 50yd. and closer. House clearing type stages with your rifle, but there's a rumor we might get one of the rifle ranges and stretch out some. Probably 3 mags will do with one or two mag pouches. For shotgun, it will be shot only, no slugs. Nothing bigger than #6 birdshot. 7 to 8 should be fine. You can carry shell carriers on your shotgun, belt, or forearms. You'll mostly shoot steel and clay pigeons. For pistol, your IDPA style setup would work just fine. If you've got a singlestack pistol, I'd bring at least 5 mags plus mag holders. If you've got a high cap gun, bring at least three. I would imagine about 150 rds. rifle 50-75 shotgun, and about 150 pistol for the match.
Regarding the courses, thats the best thing about IPSC. You won't know the course until you get there. Once again, very similar to IDPA. If you've got any other questions, let me know.