DropBox for NAS
The "File Browser" feature of a Synology NAS is an excellent alternative to DropBox. When you say " ...they're massive multimedia projects," I see a problem with bandwidth. Generally Massive is not a term used in conjunction with a cloud app like DropBox. On the other hand, I use Super Flexible File Synchronizer to sync files or folders to my Synology NAS and it works very well, but when you have files or folders that will tax your bandwidth, you end up losing too much time waiting for a sync to complete. Maybe you should investigate a remote access application equivalent to RDP, or just create a VPN to your LAN.