There are some options to explore.
Do you have a stretched LAN or do you maintain different IP addresses across sites?
We want to HA to be enabled between the two sites in addition to on-site Hosts. Whats is the best vCenter server and PSC deployment topology we can follow?
So far I researched, creating two vCenter servers (each in one site) with external PSCs (again each in one site) will form Enhanced Linked mode and replication/HA between sites would be possible.
There is no such thing as cross-site HA with two separate vCenters in separate datacenters, at least not with vSphere. Enhanced Linked Mode is only from a manageability standpoint and not a DR or availability standpoint. To achieve site-level redundancy you'd need another product like SRM, Veeam, etc.
This isnt really true at all.
Two separate sites with, two separate clusters, = Enhanced link mode all day long.
You can manage the whole lot from either of the two vCenters, and if one goes down, you still can manage and have access to the other cluster.
Its not true "HA" but easily the next best thing IMO.
Not sure what you're saying. Are you suggesting taking down one of two vCenters that are in ELM is HA as long as you can manage half of the resources? Because if so...no, it isn't.