How easy is it to create an N64 or SNES portable?

How long will it take? Are there any good tutorials out there?

Try reading this Instructable. Although I would recommend an Emulator installed on an Android or iOS device. You can hook up a PS3 controller to Android with Sixaxis Controller, a $2 app from the Google Play Store.
Your choice. 
If you have to ask the question, you wouldn't understand the answer. Basically impossible
It's not very easy at all. You gotta have a good understanding of electronics so you can condense the system as much as possible. Possible moving parts around to accommodate a better portable form factor. Then you need to ingrate the controller and some sort of screen. All the while you need to consider the power consumption of everything and find a suitable portable power source to run it all.

But on the bright side all the work has already been done and you can buy a hand held portable NES/SNES  game system for $100
http://www.thinkgeek.com/product/ed6b/