Online Converter – factors to consider prior to development

I have experience in the frontend work (HTML, CSS and jS). I worked with PHP and MySQL. And I experimented with UNIX-based webservers (Apache, nginx) and acquired knowledge with bash scripting. In an upcoming project I am going to connect these skills.

I’d like to create a website, where a user uploads some files to the webserver, the server does some magic (conversion using ffmpeg, generation of an XML-file, zipping of said files and encrypting using openssl). The user may download the output. The files will be stored on the server for a couple of hourse until deletion. Concept for easier understanding

The final product is a somewhat simpler form of platforms such as You may think the project is pointless, but I am trying to expand my knowledge through this project.

Generally speaking, what should I take into consideration before jumping into this project? Which problems may arise that I am not aware of? Which technologies/languages may be useful to pick?

More specific insecurities: As multiple users will be uploading files and expecting conversions at the same time, what is there to consider? Which technology to use, to generate and redirect to a download page?