If all you need is to view power points that is already created, consider the free power point viewer from Microsoft
2003 Power Point Viewer
2007 Power Point Viewer
Other suggestions would be to get current versions of Microsoft Office Pro for all computers and one for the shelf
Another software to consider is Map Point, which among other things could create maps of the neighborhoods where
DV occurs with pins marking locations, which you could overlay police report locations and incidents with your clients.
If you just want to get a minimum order, look at Microsoft Server CAL (Client access licenses) they are only $2.00 each.
The Vista or XP upgrade software is also handy if you get a donated computer with XP Home on it.
Do consider your potential growth for the next year as you have the order restrictions on how often you can order from TechSoup.
Also if any of your computers came with an OEM version of office, I would replace it with the charity license from here so you have transferable software when you update your hardware.
Dave