having a backup of 'sites' folder and database backup is all I need? I'd to be sure that having a backup of 'sites' folder and database backup is all I need to restore a site?
So long as you know which version of core you have and it is un-patched then yes.

If you are backing up sites, make sure you back up any other databases. You may have multiple sites under there.

One good way to test is to do a restore on another server and check things work, after all unless you know your restore works a backup is worthless.

I have found Drush make scripts a handy way to make sure that the core and modules that you use are correct. That way you only need to back up the make script, the site directory itself sites/yoursite.com and the database.