Does anyone have any tips on creating a 2.5/3D side scroller shooter?

We want to produce a game for the xbox1 or PS4, much like metal slug with shooting and platforms to traverse. Our questions are more specifically:

Budgeting the polycount across the models, I.e. Characters, enemies, weapons, level, assets, vfx? We've been hearing a lot of different numbers being thrown around and we just want to be sure.

Can the level be one huge model? or is it better to split it into components?

When animating the models, the characters will be holding different guns, what's the best way to prepare for that?

If people have any other advice, please provide. All answers are appreciated.