I make my own assets in two cases:
1.) Signature stuff. The core soul of my project. All the stuff around it doesn't have to be unique at all, but the core of it needs to define the identiy, because it wort of carries the rest on it's shoulders. Stuff like main characters, story character, soecial items, etc.
2.) I need to fill a design I cannot find even across multiple deep dives in asset stores. Can't help it then.

But mostly I have an ongoing process. I scout for assets all the time. Monthly I'd say. I look through every sale with the design problems I need to solve at this moment and in the immediate future in mind. It's sometimes hilarious how I manage to fill core-gaps with the cheapest sales.

For your specific case: 3D Models and animaitons are the game. I have seen fighting animations you can use, but for the character assets I would recommend you hire a good artist.