Hi Steve,
I don't know Zerto but I've used SRM & Veeam.
If storage replication is no supported by your HP MSA I think that there is no point to use SRM as it's power depends on storage.
I've used Veeam many times and it does not require many resources. You can do many replications at the same time. All depends on your site to site Internet link.
Regards,
Filip
