Best not to touch the OS or any special files as the Mac may not boot or work again without a reinstall.
There are several solutions:
1 - easy:
Move most contents of user folders but not the folders
Move any other user data
Move Developer folder if it exists
Move a few large Application
Move 'System Folder' (NOT System folder) if it exists
2 - better but take care with this:
Also delete non-critical caches - AppleJack is a safe way to clean disks and remove some caches
Also delete /Users/user-name/Library/Caches
Also delete non-critical log files especially:
/private/var/log/daily.out
/private/var/log/weekly.out
/private/var/log/monthly.out
/private/var/log/system.log - all items with this prefix
Use Monolingual to remove unwanted items
Manually remove unwanted modem files, printer files and any remaining unwanted languages
3 - best:
Use Carbon Copy Cloner to clone the whole disk to an external disk
Erase internal disk and do a minimum OS install
Perform task
Reinstall normal OS from clone
Keep clone as backup