you can download the Modding SDK from here :
Unzip it anywhere on your PC, and read "ModdingSDK\HowToMod.html" for instructions.
The SDK comes with :
- "NoSpiders" Mod, which changes all Spiders into Skeletons
- "MyPortraits" Mod, which contains all the built-in Portraits, so you can change them or add new ones
You may want to have a look at the Translation Kit if you want to change the dialogs or interface text : http://steamcommunity.com/app/409450/discussions/2/492378806378223762/ .
This SDK gives you access to a lot of text files used to configure everything in the game (abilities, talents, mobs, items, etc...), and most of the GUI textures, plus instructions to add your own Sound FX and a few other things.
At this point, it's not possible to add 3D models nor change their textures.