Currently we are a full Samba3.x/OpenLDAP domain. We have approx 200 users. I am testing the feasibility of migrating to a full 2012R2 w/AD setup. I am wanting to build in some redundancy to the network when I do this. We currently have our main campus and 3 outlying sites that are connected with point to point T1's. Currently each outlying location has a samba3.x BDC as their home and local share server. On campus we only have one DHCP server per vlan. I plan to leave our web/email/owncloud/etc as linux machines along with the primary DNS servers. Everything is fully virtualized on VMWare ESXi/vSphere/vCenter 5.5.
Here is what I am wanting to do:
Have highly available DHCP/AD/file servers on campus, offsite isn't that big a deal as long as we have a proper backup
Move all desktops to a single VLAN
Have the desktops DNS server(s) be the DC(s) that then can talk to the existing BIND DNS servers for outside and server entries and update the DDNS entries for the desktops.
My problem comes in the part of how many VM's will I need. I don't want to overload the DC(s) with everything but don't want to pay a bazillion dollars for 20 server 2012R2 copies. What all can be safely combined without ill effect and still have high availability?