I wanna recommend obvs model blenders and graphical designing programs that I personally love to use
Modelling:
Blender - Blender is amazing and is the much easier way of designing/customising .obj files etc, meshes and textures can be autodetected and inserted at the same time!
Clearly, the almighty photoshop!!! Paint.net (not advanced) - can be free or paid depending on the product qualityhttps://www.getpaint.net/download.html
Thanks, reply with anymore questions about suggestions and I will try to edit this with more info <3 y'alll