Depends on which version of 5 you are on. Version 5.6 isn't upgradeable, 5.7 is though. You can migrate with version 5.6, but depends on the add-ons, block types and themes you got if you have to do much extra work to make things work. Everything from 5.7 onward, you can go to version 8. Be careful though that some add-ons may not work on specific versions, because of core changes of concrete5 or a specific PHP requirement which may not have been the case when installing/upgrading your concrete5 website before. Always make backups because of this (files & database)!

The site is using 5.7.5.13.

Not sure I can see any blocks that won't work with 8 on the website.

Leave it until the update process is 100% reliable. At present, users are having issues with upgrade to v8.4.1 that will hopefully be resolved with v8.4.2.

Whatever you decide, test the upgrade first on a dev clone or staging clone, not on the actual site.

Never had a problem with updating to 8.4 or 8.4.1. But then I don't use the automated scripts or the 'updates' folder. I just rename the current 'concrete' folder and replace.