since I have the scene with all the textures, if you advise me step by step how I can proceed with post render effects much more accurate and realistic.

Build it together step by step

First problem.
The type of image for the env sphere
which picture you recommend?
Possibly with a link to download

Second problem

env sphere setting

third problem

Sun light settings

Poser Pro 2014 SR 1 on Windows 7 64 bit
I use IDL, Gamma Correction and EZSkin for all final renders.