First, we need a way for the NPC to know who it should attack.
You can create, destroy, or modify anything.
As far as gameplay goes, you can do anything in Garrys Mod game. It is a great game with a simple yet effective building system, where you can create all sorts of creations, including buildings, vehicles, weapons, and characters. I can't claim that everything i write now is correct but it should point you in the general direction! Good news Gmod download free option is available.
The additional parts can have custom textures, higher resolution skins can be used (Steve skin from Soartex Fanver - Vanilla) Parts of the normal player model can be hidden or moved around. Upload the exported image file as your skin to load the custom model. Source works a bit different than i guess you understand it. The models are stored in unused space on your Minecraft Skin, or as a Gist on GitHub.
