Remington 870 is more than adequate for a beginner to the sport. I shot my entire first year (including my first MGM Ironman) with an 870. That being said you will be behind the guys running autos. If you already have an 870 I suggest using it until you feel that it is holding you back or you can afford to upgrade to a semi auto.
The keltec KSG will put you in open more than likely, and in open you will be competing with saigas, akdals, and speed fed semi autos.