Illium is fine but you will find a link to all the trusted battery vendors on Mooch's blog which was posted.
I am a fan of Holm Tech batteries in the larger sizes, but yes to the best of my understanding they are rewraps so specs can change.
I don't think one battery is going to make your mod more waterproof than another. More EXPLOSIVE perhaps,. but not more waterproof.
You do want a flat top battey, yes.
There is no "best" battery for any mod, if you are a low wattage vaper you might do better with a longer runtime, if you want high wattage vaping you will want a higher amp rating . All kind of confounded by the fact that these batteries are somewhat new, there aren't as many "best authentic" choices in comparison to say and 18650, so gosh, I'd probably go with the one that best suits your needs as far as wattage, as best as you can determine..
Good luck and welcome,
Anna