You should reconsider a NAS because using a Mac Pro is complete overkill when a NAS will do exactly what you want including sync to public cloud services like DropBox, OneDrive, Box.net, etc.
I am a fan of Synology products because whether you are using their entry level single disk NAS devices or their enterprise grade rack mount solutions, the software and features are the same. Specifically, to your question, it includes Cloud Sync which will sync up with DropBox.
Also, consider moving from AFP to SMB. AFP has been deprecated in Yosemite in favor of SMB2. AFP is also not compatible with the new APFS.
However, most NAS devices will still provide AFP for backward compatibility.
Finally, I suggest you utilize their "NAS calculator" because sizing a file sharing solution is more than just how many people it needs to serve, but the type of files, how much space, your iops requirement etc.