The free versions are called Standard versions, as opposed to the paid versions called Advanced versions. The entitlement requires CUCM 10.x and later, using CUWL licensing and is covered in the CUCM sales literature.
There is not a lot of difference between PCP standard and advanced by design. The most common reasons to choose advanced is if you have a multi-cluster environment. Standard only supports one call processor (CUCM publisher or CME) and one message processor (CUC or CUE). The second most common reason is that in Advanced you can delegate order management of different groups of users (PCP Domains) to different admins. Standard mode does not allow this. There are some additional differences like the northbound API is on in Advanced as well as an order workflow process sometimes needed by large enterprises. Those are the main differences.
HA is another story. There is a general HA process supported by the BU for PCP and PCA using VMWare HA. HA in general has not been popular with PCP because most people don't believe it is necessary and rather choose a different path for resilience. Most people deploy in a distributed mode (app + database VMs) and make the database system as fault tolerant as possible/needed using typical industry database processes. The App server uses VMWare HA or FT to make it come up if the server hardware dies for some reason. Or it is just launched from another location.
PCP has UC network re-sync capability, the UC network will continue to run without PCP running, and the UC apps native interfaces can still be used (PCP can catch up with the changes later), the need to expend extra money and time to setup HA is hard to justify in most cases.
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