first off… n00b alert… .please be gentle!! 
I need to update my install and since its my first update I don’t want to mess anything up. Ofc I will perform a backup first… is there anything else I should be aware of for my first update? Is the site automatically taken offline? Is there a message generated to users to that effect?
Just trying to avoid the oops message. Any pointers would be appreciated… so TIA!!
A backup for sure, and even if everything should be fine, if you have some kind of vps feature for that, maybe a snapshot of the host 
, as surely there will be some database update to be done, might be easier than to restore from a backup if anything goes wrong (restore process that is interesting to practice by the way). Make a copy of the app.yml and write somewhere the SHA of the commit you’re on as an extra precaution ?
2.4 to 2.7, there are some postgresql updates if I’m not mistaken, maybe browse some of these Search results for 'postgresql update' - Discourse Meta as a warmup 
Yes, the container is stopped during the rebuild
Nope, not out of the box at least
Another question… does the update run a scan of the installed databases and plugins to make sure everything is up-to-date before commencing with the update?
Yes, when rebuilding a container, the launcher pull the latest plugins from their git repo. If you made some modifications within a theme or a component, you’d want to save these changes