you have multiple options to achieve HA, let me start with the recommended one :
a) create two virtual ESA gateways, use Ironport application clustering to have two permanent running ESA
b) create a second ESA in the cluster, shut down second ESA and only bring up 1 a week for updates and when needed.
c) save the config of the existing ESA with password, Define a process how to quickly spin up a new ESA and import the config.
d) clone your existing ESA (when shutdown), bring up clone when needed. Starting clone will take some minutes due to required updates and cloning should be repeated in regular intervals to keep config updated.