for the average office
What is the "middle office in a vacuum"? How many users at least?
The domain controller
I searched, read, there is an option to start on Debian
It is not necessary. Normal PDC on Samba not to do, you still Win not to get to anywhere, because seven of odines and consultant. Because the domain controller do to win, users win because sit, as I understand it, why else would the controller?
The main burden will be to create a 1C, I would not be the city all on one server as a minimum reserve need. You need to take two servers with lots of RAM and to lift there server virtualization (ESX, Xen, KVM), one server is primary, the other as a backup.
Option number 2 is to take four physical servers easier, on one pair the Linux farm, the second pair of Windows, or simply scatter. Single more powerful server, for 1C base, others are simpler, because DC, Filipovich, the consultant and the rest a lot of resources will be required.
Take one big server and pile on him all at once — wrong. In the event of failure of its hardware or software logic (OS will be covered or admin makes a mistake) you'll lose all the services simultaneously and permanently.