I would like to hear your opinion on a project I'm trying to do.

I Have a Teensy 3.6 with an SD card. I would like to edit text files that are on that card - open a file, display contents on a display, add letters, delete letters, possibly copy and paste fragments, etc.

My problem is that the files are max. 50KB (50 000 letters) so I would like your opinion on how I should start with a data structure to handle this. The Teensy 3.6 should have enough RAM to just load the file and do operations on it but what variables, arrays, etc. should I read about to accomplish this? What would be the most efficient way to approach this?

I have been using things like Arduino UNO for now making simple projects.

Thank you for any suggestions.