I would like to know how easy it can be to do such modifications.
If you know C well enough, then after reading about 5000 lines of code searching for what to change, it'll be quite easy.
Do i need to recompile anything (is it part of the 'scripted' thing?)? the whole game? Only a part?
The only part of code which should be interesting for you is in the game directory (code/game/* in SVN). qcommon and server directories might be a little interesting too but only if you can't do something in game alone. Make is smart and won't recompile things which don't depend on anything that hasn't changed since the first compilation. You can also turn game client compilation off manually (make BUILD_CLIENT=0). Server executable might require recompilation to match game.qvm in some cases.
Do you have any link to a tutorial on this subject?
Code3Arena should be a good place to start.