vSphere ESXi 5.5 environment - updates and maintenance mode question

I am looking into updating esxi hosts in an environment that's gone unmanaged and non been updated in a while.
I believe there are many updates needed for the hosts, as shown here in this screenshot: https://i.sli.mg/HCi6w7.png 

My understanding is that for updates, it's best to put the host into maintenance mode, so that the VM's go to other hosts.
What's the best way to ensure that is configured properly, and that there would be enough CPU/RAM on destination hosts the VM's migrate to to function while the host is in maintenance mode?

I'm assuming given the volume of updates available, that they all need to be installed, and will require multiple reboots...