The typical corporate setup for something like this would be something like this.
MS Active directory server.
MS File Server
MS Terminal server with Citrix XenApp.
Active directory controls users and groups
Fileserver holds files for sharing. You choose what users have access to what files/folders.
MS Terminal server allows users to access there applications and files/folders remotely.
Citrix XenApp enhances the features of terminal services with there ICA connection protocol, it allows users to access all files they have on the shared drives they have access too and they can also stream applications such as microsoft office, adobe photoshop, so on so forth. As if they are on there local machine. Because of Citrix ICA Protocol it is possible to do this with minimal bandwidth (by minimal i mean 56k modem speeds.)
This is what most mainstream companies use for remote file and application access. Not sure how many employees you have. Not a cheap solution, but it is the industry standard for small, midsize and enterprise level companies.
It is not cheap though
