I don’t disagree with the recommendation to go shoot it and sort out what you’d like.
That said, there are a couple things I’d strongly recommend, having built one for myself last year:
1) A good sling. The Blue Force Gear Padded Vickers sling is a game changer and the best $50 upgrade possible. Yes the gun came with a sling but it doesn’t come close to working as well
2) HiTechcc bottom rail reinforcement and a vertical foregrip. Protect your hand with a positive grip.
3) Kydex/Neoprene cheek wrap from MCarbo or HiTechcc
4) Sights can go to any budget. Magpul MBUS are a decent inexpensive option; if your life is going to depend on it, the Holosun HE510C is a great optic well suited to the KSG
5) Consider a weapon light. The Streamlight TLR RM1 is a good option, but may be brighter/more than you need
6) The LaRue Tactical Covert Rifle Case fits the KSG like a dream but costs $200...it makes it imminently portable yet “discreet”
Holler back if you need help finding these with links, and mostly go practice and train and have fun!