1. Would it be harder to build a portable snes with the older model, instead of the mini?
2. Is there a guide/tutorial in the book that will explain how to do this?
3. What kind of casing/material should I start with for my first portable snes? I dont have much background in soldering or tinkering with electronics, but Id like to try
here is a really old pic to give you an idea, if you get the model with the separate APU and decide to relocate it and the cart slot to save space you are going to be doing a LOT of soldering 
if i were you i would go with one of the Chinese clone super Nintendos, they are small and reasonably cheap now, and there is a ton of information on how to modify them in the stickies in the SNES section