I shoot 5-stand with an 870 Express. But the pump is a real handicap when shooting pairs. That doesn't stop me from hitting both, and it's good practice for shucking fast and with finesse. If you can REALLY use a pump, you can use it to shoot pairs, and vice versa. All these people who talk about short-shucking just haven't practiced. With ammo that feeds well in the gun, and a little oil, it's never let me down. But I'd be lying if I said I could score the same as with a double gun.
I shot trap with the thing, too. A pump that fits you will shoot singles trap as well as anything. But I recently got a BT-99. If I get a gun all modified to be perfect for singles trap, I figure it should be a trap gun, not a hunting beater like the Express.
My recommendation would be a semiauto that fits you. If an 1100 fits you, a used 12 Gauge with 28" barrels ought to get you into the sport, and there will be no good reason to buy a new gun for a while. Buy a high-buck gun when and only when you've done enough shooting to know exactly what you want!
Note that a full-length field gun works great for Sporting Clays or 5-stand. Field guns may not be the best for trap, and skeet favors shorter guns, but a regular old 28" 1100, 11-87, Beretta 39x, or similar run-of-the-mill semiauto will work well for SC. Same goes for a 686 or other common double gun.
If you get into the sport and you have the money, you may well want a $2400 gun (or, sickeningly, a $12000 gun) one day. But at least you will know which one you want. You can buy the wrong $2400 gun and wish you'd laid down a few hundred for a used 1100 or 390 instead.
Good luck and have fun!