Seeing that you mentioned they have pirated versions of Windows XP, I would assume that the cost of a Windows Server 2003/2008 license with enough seats to take care of the 35 (and probably more in the future) computers and the Windows 7 licenses, not to mention the hardware that needs to be purchased, would not be a viable solution to your client.
I've worked in places where they expect The World for free. They do not want to spend money.
If you can persuade them to upgrade to your specification, then you should have a nicely working system on your hands, but squeezing the money out of those kinds of clients is difficult.
I wish you all the luck I can grasp!