Building solution for a network server

Hello all.

I have to build a solution for a network of around 150 users from the same building with about 50 users per floor.
They are all having an workstation with windows 7 prof (about 75%), windows XP home edition (about 20%) and windows xp prof (5%).
They are working in Autocad and I need a server on which I have to put windows 2008 server with AD and will have a role as fileserver.

I need to know what are suppose to be the minimum hardware requirements for this server.
Having not too much IT experience I need to know if the users that have windoiws xp home edition need to change their OS.
Do I need to know anything else before I have to give a solution?
Thank you very much.

Best regards,
Michael