Thank you for posting question @PZemtsov
By cluster do you mean active/standby or active/active failover pair? If yes, then there is no impact/disruption while building failover pair. There could still be an outage in the case of misconfiguration or mis-operation. For example if there is an failover to the other VM where interface mapping is misconfigured.
After you complete the HA configuration, I would recommend to make a failover test that would just in case tolerate downtime.
If you mean clustering function introduced in PAN-OS 10, then lowest spec VM supporting this feature is VM-300.