I had a look at AeroFS for similar requirements, it runs as a vm, can be pointed to a NAS or other file shares, has apps and the syncing seems good but the problem was to get external connections / syncing you had to point 443 at the AeroFS Vm and we use 443 (ssl) for other things. If you don't use 443 and can create an A record pointing to your static IP then it could work well! (-;