How to competently organize the creation of backups from user computers on FreeNas 9.3?

Question

System administration

Hello. I decided to set up backups from computers (OS Win XP/Vista/7/8.1, about 30 pieces) to the network storage using KSOS 4/5. It is supposed to copy only working documents, so the volume will not be too large and the tools built into this antivirus should be enough. To begin with, I selected 2 network balls (for bases and working computers) and created 2 users with access rights to them. In the process, questions arose regarding how to organize everything correctly:
1. Is it enough to have 1 user with access rights to a network folder for all computers, or does each computer need to create a user and allocate a separate folder (inside the network folder);
2. Use encryption (each has its own key or 1 for all) or not, if any of the users gain access to this resource, then they will not be able to use the encrypted data, but can simply delete them;
3. How to limit the resource allocated to each user (for sure there will be someone who will put not only working documents into the folder with which the backup is made).
Perhaps there is a more flexible and convenient solution for the implementation of the above, I will be grateful if you tell me how everything should be.