What is the minimum number of modifications in order to install a custom ROM?
To install a rom you have to root your device, there is no way without rooting your phone as long it isn't rooted yet
Sorry dont understand your second question. 
You have to flash a custom recovery such as Clockwork Mod Recovery. You need a custom recovery because you can't control the default recovery (no way to acces it). So you have to set your phone in downloadmode and flash the root and the recovery. 


Have a loook at this step by step  tutorial where everything is explained ( http://forum.xda-developers.com/showthread.php?t=2215996).