System requirements to install windows 2012 for 150 users

Dear friends,

Here i planned to implement the windows domain module. Totally here we have 70 hosts for 150 users. We preferred to use windows 2012 as a DC. We may use the roaming profile for users. So all the user profile will be store on DC itself.  

But i have a no idea about which configuration system we have to buy? like processor speed, ram size, HDD size.

So please suggest us to make a right decision.

Thanks,

Lingaprakash R