You'll need an inner/outer belt setup like a BladeTech, Double Alpha, CR Speed or Safariland. I've used blade-tech/DA/CR Speed since I started.
Then you'll need a holster and 1-2 pistol mag pouches. I like BladeTech pro series pouches and WRS II holster.
Then you'll need a few shotgun caddies. Invictus practical are the ****. If you've got the funds get a 12Q, 8Q, and a pair of 4Q's. If the budget is tight go with one 8Q or 12Q.
You don't need a rifle mag pouch but they are handy. You could get a mag coupler instead or runs backup mag in your back pocket.
If you're in it to win it grab some TMMS mini clips and make the whole system QD.