Paranoidox

The proper answer for this depends on your non modding related background. If you've never even touched a 3d editor and nifskope, you'll likely have a hard time. If you happen to have a strong background in working with game engines it'll likely come very quickly.

You could probably use F4SE and have a script that changes the FOV when a key is toggled. I'd recommend leveraging the "fov #" console command if there isn't a function for it. I Know there is a way to check what mods are on a gun if you want it to be scope dependent.