I guess the only couple of question are: do you want SSO (Single Sign On) with the hosted email? eg Google authentication, OpenID, etc... Number of desktops, OS of the desktops...
For centrally managed setups, you can either run a single Windows Server acting as a DC, DNS and DHCP server and have it perform file sharing. (single box doing it all). Also remember Windows 2012R2 Data Center edition can run unlimited number of Win2012R2 VMs as part of the license if you want separation of concern, you just need a box beefy enough
You can look at Windows 2012R2 Essentials, which IIRC is the replacement for SBS and provides direct integration with Office365...
Alternately, SAMBA4 can act as a single DC (IIRC, it emulates a Win2008 DC), but you don't get GPO support, and there are limitations on sharing and authentication functionality. (It's all the samba docs).
Or as dd mentions, run a NAS for central file serving, and run network printers so you don't have to worry about permissions between clients...