Have a good zero on your rifle. Don't just zero at 50 and expect to be good at 200. Zero at 200 (with most optics), and check your hits out at 3-400 and 5-600 if you can get that far.
Ideally your shotgun will hold at least 9 rounds. Mine has a tube that holds 12 just to simplify loading plans right from the buzzer but 9 is enough. Every once in a while you will come across a big shotgun stage that will need 40+ rounds but those are not an every match occurrence and someone will lend you a spare caddy or two if you need it. Check out the quad load caddies and/or vests from Invictus Practical, Carbon Arms, Taccom, etc. Most of the time 9 in the gun and 20-24 in caddies is more than plenty.
It is helpful to have a couple of extended mags for your pistol but not necessary. 1 or 2 TTI basepads are nice but you do not NEED them.
Practice. Practice offhand rifle to 50-75, practice hitting little targets with your handgun, practice NOT MISSING with your shotgun and practice loading your shotgun. Get some dummy rounds and you can practice loading your shotgun in your living room.
2 pistol mag carriers is almost always enough. 1 rifle mag carrier is plenty and honestly a back pocket won't slow you down much on rifle reloads.
Get a decent optic. You don't have to break the bank but you don't want your optic taking a crap on you. In my opinion, reticle design is just as important as glass quality and reliable zero. There are some decent scopes out there that are total deal-breakers for me because the reticle is so bad. The Burris XTR II 1-5x is a great value. I have a Vortex Razor and it is superb but a decent chunk of change. If your optics budget is big a lot of guys swear by Swarovski and Kahles and they are fabulous optics. I just got a Steiner T5Xi for another rifle and with about half an hour of trigger time it appears to be very good but it needs a lot more vetting before I will know for sure.