Budget file server with access from an external network. Where to look?

Question

FreeNAS

The situation is this. There is now a server on Win (not persons.). There is file storage. We planned to change to FreeNas for storage + Kerio Connect (including for remote work). As a result, the management did not agree to finance the solution and left the WIn server.
At the same time, we have a group of users (so far 5 people) who need constant access to files, incl. from an external network, so we (this group of people) used the shared OneDrive for Business folder, as a result, due to a number of problems, we switched to Yandex Disk. With poison, too, there are difficulties.
The question is, is it possible to organize an inexpensive solution for organizing storage and working with files (Excel, Word, PDF, images, etc., nothing special - documents, currently about 10 GB, about 8-9 thousand files ). It is important that the storage works in the same way as a local file server, for example, that at the same time a separate file is writable only by one user who opened it (I don’t know how to call it correctly).
I would very much like to get advice in which direction to take a look based on the criteria: ease of organization and low price of the solution, and of course reliability (reasonable, super-security is not needed, because we do not store codes for launching missiles).

Answer 1

VPN to your work network and don't bother.

Answer 2

FreeNAS as a file server. On a local network, this is access via SMB / NFS.
And to it NextCloud to it - for access from the web via UI or WEBDAV.
How about blocking files - I don't know.
NextCloud has Collabora, a shared online document editor / viewer.