existing AD server and azure 365

Hi All 

I have just taken over a site and I have one AD-DC and emails are on o365, if I go into azure ad 365 I can see all my ad accounts. 

I would like to install and set up the ad sync mainly for password sync between local Ad and the email accounts. currently have about 20 users, and rapidly growing so would like to get sorted sooner than later.

if I install the sync tool, what am I likely to break? what do I need in place before I setup and start syncing... I assume the password for email in o365 will change to the local AD passwords..

would like this to go nice and smooth as possible, 

thanks for your input

Cliff