This can't be overstated. I've lost count of the number of lawndarts I've seen due to flying stock ejection charge in a 4" or larger airframe, and I've only been attending high power launches for a little under three years.One thing to check: ejection charge. The bigger the diameter, the more volume the ejection charge has to pressurize. Might need more than the standard amount of black powder eject charge that comes with the engine.
This also can't be overstated. Fly what you want to fly on the fields where you'll be flying it. If instead you build a cert-specific rocket that you won't actually enjoy flying, just to get the cert, you'll be wasting your time and money.(People who like 'em low and slow are the lucky ones here. )Fly what you like to fly. A cert flight is nothing more than any other flight, but it gets witnessed. If you post styles of rockets you like, you'll probably get some good, honest suggestions.
Like others have said, the satisfaction of a scratch build is nice too, but whether kit or scratch, build something you'll want to keep on flying.
DD can often be cheaper, but JLCR sure is easy to add to a rocket.I used to call the Jol;ly Logic Chute Release the "Poor Man's Dual Deployment, until I bought one.