Given only what you shared, why do the buildings need to be connected at all?
And if all on on one LAN, then the server is on the LAN and there is no need to add more servers unless you are like me and have a test server to try out new code and apps.
As to the DNS, your choice. What ever your DHCP source is can hand that out.
If I have three buildings in my LAN do I need a server for each building? If not, how do I set up my network to share a server? Also, if I have a domain name how can I host DNS?