Is this Backup System sensible?

[ScratchCat]

I would like to set up a backup system for multiple computers, mostly 3 with 250GB drives each (Approximatly 50GB of user data each).

Currently the system works as follows:

1. Each computer backs up to a shared folder on a 500GB HDD connected to a Raspberry Pi. Each computer only has the login for its share. The backups only contains user data and so all 3 computers will fit onto the 500GB disk.
2. Two offline disks (1TB and 1.5TB) are used to store system images and copies of the backups as well as other backup data.
3. Offline disks are alternated every few weeks and only are connected when creating backups.
4. One copy of a system image is kept on each offline disk, any old copies are deleted.
5. Backups are completely redone every 2 months, keeping one offline disk with the old backup version ensures that previous backups are available.

Note: Other HDDs are available however they are <320GB and mostly have pending sectors and hence will only be used as an additional copy.

Diagram:

 

Is this a reasonable backup system and can it be improved in any way?