I'll post similar suggestions to what I've posted before in other threads like this one.
First of all, build a rocket that you love. You want nearly any rocket you build to be something that you want to fly (or at least love to look at!). Sometimes that's a more extreme choice with a higher risk of failure. That's OK, as long as you go in with your eyes open about the consequences of failure and do everything that you can to mitigate the failure. Any risk should be for failure of your certification rather than danger to the public.
With that out of the way...
Just about any kit with a 29mm or 38mm motor mount will be a fine certification rocket, with an appropriate motor choice. Choose just about anything that you like. Cardboard body tubes will give you more flexibility in motor choices since you won't need as much thrust off the pad for a light rocket. The kits also tend to be cheaper.
Body tube size: A cardboard 54mm rocket on an H motor will be on the hairy edge of going out of sight. If you lose the rocket, you don't get the certification. 2.6" to 4" rockets are very popular for certifications. They tend to fly lower and be easier to find. Larger than 4" and you'll have to think a little about how heavy the rocket is for available motor choices. Think a little about what waivers altitude you have at your launch site. If it's 3000 feet or less, you'll want a larger body tube diameter so it doesn't go too high. Likewise, if your field is surrounded by rocket-eating trees, you'll want to fly lower. If your launch site is clear ground in all directions and a waiver of 5000'+, those considerations don't matter as much.
Motor mount: If you're on a budget for this and future flights, use a 29mm motor mount. If your first thought on getting the certification is to ask how much more motor you can put into the rocket, get a 38mm motor mount.
L1/L2 rockets: If you build a larger rocket with a 38mm motor mount, you can stuff a J in there and get your L2 after you turn 18. I am personally not a fan of this, but it's a valid choice if you have a more limited budget. On the other hand, if you have a limited budget, getting an L2 might not be the best move anyway.
Fins: Fins that don't extend past the end of the body tube are less likely be damaged on landing. Excessive damage = failed certification. But choose something that you love. Three fins, four fins, twelve fins, it's all good.
All of these tradeoffs apply to scratch-built as well as kits. I'm personally a fan of scratch built (especially for L2), but again do what you love.
Good luck!