Windows-95 is all you need. 98 is designed to handle home applications that you'll probably never use. In a few cases you might need 98 or better for your hardware. Probably you already have a copy of the OS and simply need to call Microsoft and pay them for the license to install it on another machine.

Windows-NT4 or Windows-2000 are just overkill for you. They impose an extra burden on the server structure which may or may not require you to upgrade your server structure. If it does then it is a hidden second cost that your company needs to be spared from.