I have taken over an outdated development project with no idea where to begin

I was doing some product design for a small company and the head developer and his team have gone AWOL. Now I need to hire a new development team myself but not exactly sure where to begin this takeover.

I've spent the last few days looking through the code and figuring it out. It's an e-learning web application but how it's built is quite outdated. It's built using Javascript ES6, PHP 7.0, NodeJS 10, MySQL. No frameworks used, not responsive on mobile devices, no git or any sort of version control / tracking, hardly any documentation at all, state management is weird, and probably more issues to uncover. The website works fine and does what is required – but to add features and modernize it I'm a bit lost. I have to start hiring developers to start adding features in the new year but I don't even know where they should begin as I cannot do a knowledge transfer in this state.

My question is what should be the process here? What can I do to support a development team in this situation. It requires a lot of info that we cannot get from the person who built it in the first place. Would it make more sense to start from scratch? Any help would be greatly appreciated, this is stressing me out.