VLAN to Flat networks

Currenty we are using the VLAN technology of cisco but next year we will be shifting to a flat network. We are on the planning stage of our flat network design and configuration. I just wanna know what are the implications of shifting to a flat network. Maybe, if anyone of you had already made a comparative study regards to this matter, I hope you can send that to me. This would help us a lot. As of our last inventory, there are at least 2500 users in our network.
We’ve consulted a microsoft engineer and he’s the one who recommended to us to shift to a flat network and use DHCP, to solve IP addressing problems, single “sign-on” and ease of network mageability. Does cisco have the technology to match these ones? Thanks!