Doesnt sound like you need XP prof. Back in iima, we did all kinda thingies (office/email/websites/macros for excel) - XP home did the job just fine (esp since it came preloaded on the cheapest laptops).
Just checked
http://www.winsupersite.com/showcase...p_home_pro.asp - you could choose a mix. Frankly, most people don't REALLY need the additional functionality. I presume you would anyways be hosting shared folders on a central server, so people logging onto each other's desktops is not needed, hence irrelevant for you. (do check on being able to disable logins from other PCs though - for virus safety angle).
And as for office - considered open office for the people who just have to check occasional excel and word documents? Esp the lower level clerks.
IBM Lotus Symphony is there as well
PS: Sorry, just re read your post and am sure this confuses you moooore
