We have a small design practice (2-3 people at the moment), and have tried some systems / services that may be of help for your situation:
Tried it for some time, worked well, but it kept giving errors on startup (mostly on Windows) and I would have to stop it and restart it after rebooting.
On the other hand is free for local sync up to 30 users, and you get a smartphone / tablet app if you pay a fee. Versioning and conflict resolution implemented, as well as selective syncing. No server needed but you can set up one for backup / archiving purposes.
Never fully implemented it but it seemed perfect for local file sharing / syncing. You need a server or NAS (Freenas works great, it has an OwnCloud plugin and it's relatively easy to set up). It has a free smartphone / tablet app. It also has versioning and conflict resolution.
Not cheap but works great, all of the time. The LAN Sync feature is very powerful and perfect for fast updates on big files, so your low bandwidth does not slow down your workflow. The problem in my opinion is the price, although last year's price war with Google multiplied their space offering tenfold (100GB > 1TB) for the same fee –USD$ 99 / year last time I checked. This is for personal accounts though, they charge USD$ 750 a year for 5 users and 5 TB.
It has versioning (for 30 days, or more as a separate payed service), conflict resolution and selective sync.
Good pricing –unlimited storage for USD$ 10 / month which is kind of unbeatable and it's why it is our current setting. Ease of use, selective syncing (so you won't have to carry all of your archive with you all of the time but you can still access it anywhere if you need it), file versioning or history; I'm not sure how it handles conflicts. No LAN syncing so implementation is painful and sharing a big folder with a new collaborator is slow. With Calendar, Tasks and Docs it's a good candidate for collaborative working. It has an offline mode for flaky connections.
We do have a NAS (Freenas), but I find it a nightmare not being able to have some kind of version control; also, on our current setup it would be difficult to access from different locations (i.e. while not in the office), so we use it for backups –it has a Time Capsule plugin.