Resolve 18 is currently a public beta but it is on version 5 already so it may be considered to be pretty much there by now by many users and Youtube independent trainers.
Switching from 17 to 18 may be confusing for you if you are unfamiliar with Blackmagic's way of doing things regarding how the database works and how to backup restore etc. If you are part way through a few projects using 17 and then upgrade to 18 you may inadvertently screw things up.
So I would say go straight for 18 - which is what I'm using.
Any company can change their pricing plans obviously, but there have been no indications that the perpetual license may change to charges for future upgrades - quite the opposite.
These trainers among many others are using 18:
The middle link does not appear to be working direct in this thread at present but it is working in Youtube. Search for Casey Faris - he has a complete crash course on 18