Well,
using SAMBA would work just fine, but there might be security issues to consider. By using SAMBA, and making files accessible by Windows machines, there is a possibility of unautherized users (ie: scriptkiddies) gaining access to your files and raising cain...
An ftp server might do the trick, but if each user has to DL a file and then upload to save it, what if another user is trying to do the same thing? The file would be overwritten by the last one to upload...see where this is going?
There a ton of questions flying through my head, but something that might cover jsut about all of them...are you going to be using a database? If so, see what db will meet your needs. If I am not mistaken, most of the better db's can control who has what files, so someone else can't get it and overwrite it at the same time, bla, bla, bla...
As far as hardware...for 5-10 users, you could jsut about use an old Atari ;-).
What types of files are you going to be using? I mean like Word Templates for the users to DL, then they fill them out and save them on the server...or live files that are modified locally (on site) and abroad? ...sorta like a Network Drive?
I am a little shakey with WAN file shares, but maybe I can point you in the right direction...
Hope I was of some help...
--OR--
Thanks for the help...
--Rich