[jem]

Actually a better solution would be a SBS 2008 (Small Business Server) installation at head-office or wherever the majority of the users are and a Windows 2008 server running terminal services (TS) in application sharing mode, loaded up with the Office suite and any other software they use. That way you don't have to worry about flakey internet connections corrupting open documents, you only have to worry about one set of software being installed and you can use any old PCs at the satellite sites running any version of Windows (or not even Windows if you really are feeling brave as long as it has an RDP client). People can also work from home without you having to bother about what the state of their home PCs are viz-a-viz antivirus, patches etc because all these client machines are not actually connected to the office network.

If you want, you can use VPNs to connect the remote offices and home machines to the TS box but it's not too unsafe to not bother and just connect via SSL-that way you don't have to bother about VPN configurations at the remote offices

Charities can get some very nice software deals from Microsoft, (I know because I set up a similar arrangement for a charity only a few months ago and they could get licences for Office and Windows for a fraction of the normal cost).

You won't need anything stupidly powerful for the SBS but I'd recommend a 64-bit server platform for the TS box with as much RAM as you can stuff into it (and as budget allows). How many users and how much data are we talking about at each site BTW?

Unfortunately none of this is very easy to put in and configure, but it pays dividends in simplified support costs after. Oh and don't forget a really good backup system. Tape drives are the best in my opinion (not cheap though although the tapes themselves are reasonable) unfortunately MS in their infinite wisdom have taken support for tapes out of SBS2008-madness I call it.

Good luck.