I am inheriting a couple projects that were built with ACF 4.x.x and some of the premium plugins (options, repeaters, gallery).
Now that 5.x has come out will the individual plugins no longer be updated? I am not sure I can get the licensing for a free upgrade as it may be in the hands of the past dev, but should I upgrade to 5.0 (best practice)?
You can update ACF4 and the addons should continue to update if there are any updates.
If it was me I would update to ACF5 Pro. Follow the upgrade instructions. Especially the part about activating ACF5 before deactivating ACF4 and then immediately deactivating 4 and all add ons before making any changes.
I have run into problems when upgrading where field groups become unattached from options pages. You just need to go in an edit the field groups to reattach them to the right options pages.
The only other problems there could be is if field groups are added in code rather than through the ACF editor. These field groups may need to be recreated. The code used for 4 and 5 is slightly different, different hooks need to be used and some alternate function. All of this is listed on the upgrade page. http://www.advancedcustomfields.com/resources/upgrading-v4-v5/ and the what’s new in 5 page http://www.advancedcustomfields.com/resources/whats-new-version-5/.