Many things need to be taken into account here such as:1
1) Amount of data per site.
2)Total data to be uploaded across all sites.
3) Upload speed per site.
3.1) ADSL bandwidth limites per site.
4)Onsite backups (If files are needed due to deletion and you're offline you will have a problem..) Look @ FreeNAS for a good solution that can be secured behind a firewall such as PFSense. (generally you can use 2 oldish PC's for each applince or virtualise them on 1 solid modem i5 with ~12-16GB RAM)
5) Cloud backups - DO NOT map these to your local system, as viruses like Cryptolocker (equivelent of having your data password protected by criminals and then held for financial ransom). DO use somethign like Mega.co.nz, they offer 50GB free per user and have a sync extention for Google Chrome [making things VERY secure].
The lower cost option is to get a external HDD per site,(personally I recomend Western Digitnal Red or Black drives, 1-2TB per site should be sufficient. Avoid facotry assembled units like Samsung as they usually only have a 1-2year warranty, where as WD Red,s have a 3 year warranty, and the Black drives have a 5 year warranty) ) use it on 1 specific users PC and map it to the other PC's, then use some free backup software to carry out the backups and provide logs. At night lock the drive up some where safe, keeping in mind that onsite safes are the main target of theives.. This covers onsite.
With regards to offsite you can get a modern i5 with 4GB RAM, and say 2x 6TB WD red drives (assuming 1TB is enough per site) so that the data is always duplicated on the offsite unit, incase on of those HDD's fail (you dont want to upload ALL that data again).