Xbox 360/PS3 - 3500 - 15,000 256x256/512x512 pixel textures
Xbox One/PS4 - I honestly don’t even know, but it’s only slightly higher than the previous generation. 512x512/1024x1024/2048/2048 pixel textures
As for where to start when you model anything, start with the least intimidating part… Perhaps the head.
You can rig your character as long as it’s fully modeled.
Avoid unnecessary polygons/vertices and try to keep the character is low poly as possible while balancing visual appeal. You want the character to be as well optimized as possible to run better on the target system.