Hello Rhonan!
I would say that the Unity Asset Store can provide many good and free assets, with the advantage that, by being in the Unity Asset Store, they are guaranteed to work in the Unity Engine! Offcourse, this recommendation works only for Unity :D
As for "success", I understand that succeeding in a game jam would mean to deliver a playable version of the proposed game! With that in mind, I would advise tracing a realistic plan, a simple game can be a very good game! Not too many assets, not too many mechanics, just what you know you'll be able to implement in the given time for the jam that are imperative for your idea to come to life! You can always keep developing the game afterwards! And give yourself enough time to work on the project, but don't linger too much in problems that aren't game breakers! Bugs are okay in a game jam!
Hope you enjoy the experience! I surely did last time I managed to participate!