cloud file sharing solution for small company

Need some help finding a cloud file sharing solution for a small company (25 staff).

My client likes the ease of use of Dropbox and wants something similar but without the high price.

Requirements are as follows:

- automated syncing of files between office pc, home pc, smart-phone, and web interface
- different access level for different users when sharing (Dropbox fails this)
- file access via Windows Explorer
- data encrypted during transfer and during storage online

Seems it's possible to encrypt Dropbox data online via 3rd party encryption tools. See http://www.makeuseof.com/tag/encrypt-dropbox-files-boxcryptor/

Looked briefly at some NAS devices from Synology but they don't seem to have Dropbox functionality - see http://blog.synology.com/blog/?p=593

Any help much appreciated!