I purchased an iMac (Catalina) from a guy on FB and I recently found some of his old files on a Crucial 250GB SSD. I got in touch with him and asked if he needed any of them and he doesn't. How do I know if I can reformat/erase the drive so that I can make use of all the space available on it? I'm not sure if there are some system files on there or something that will make the Mac crap the bed and cause me a big headache. Any help would be appreciated.

I can post screenshots (I think, if they're not regulated on here) if that will help.

Thanks!