Well it depends on your hardware. Are all of the computers you intend to setup identical? Also what are your long term plans? Some distributions are better supported and more "professional" then others.
I would suggest that you give Conectiva 10 a try. I found it to be easier to install and administer then Mandrake 10 Official. Also Conectiva looks A lot better then Mandrake 10.
I sugges that you check out the Webmin website.
Webmin will help you setup your office LAN and servers without having to use a lot of scripts or Terminal sessions.
Both Conectiva and Mandrake support Webmin.
Well good luck on your decision making. Oh and make sure that you update your distribution of choice before making your office LAN internet accessible.