Business continuity best practice

Hi experts,
My client runs a server 2008 on a hp dl380 gen 7 rack server, they run a critical third party application on it. We have set up onsite backup to offsite with arcserve.
They have hosted exchange.
The issue i am concerned about are should the server have a motherboard failure (its 5 years old)and although we have full image backup its the 4+hour outage while we restore.
Also the operation works from 9-11pm
So patching and reboots is an issue.
So I was thinking could a solution be something like the software doubletake and have a identical server onsite,should the main one fail it kicks over(also I see they have this asan azure option).advice appreciated.
Thanks