I need to add a drop-down menu functionality to my Plone 4.3 or 5.1 Ubuntu 16.04 VM installation. Where to find a virtual appliance with all this loaded already?
What do you mean with virtual appliance? If you need some add-on in your Plone installation, added to your buildout configuration and rerun buildout.
https://docs.plone.org/manage/installing/installing_addons.html