Hi!
My name is Sergei, I am the Microsoft Office 365 Certified Solution Expert with over 17 years of experience in system administration.
I am a quite new here, but can be valuable for you..
Do you have Office 365 already set up? If not you have to have access to your domain DNS zone before starting the project.
Migrating your mailboxes is easiest part, as you have only twelve it can be done within a few hours.
Skype for Business is more complicated question if you want PSTN connectivity.
There are two ways - replace your on-prem PBX with MS cloud PBX or set up separate Skype for Business server on your premise and link it to your PBX and SfB in cloud. My suggestion is first option, it’s a bit more expensive but you will not have problems in future and you will not be dependent of your local PBX (you can move office everywhere without telephony downtime).
Hope to fruitful partnership.
Have a nice day!