Rear bumper installation and sag question

[JRHernandez88]

Whats up guys I just bought my first truck and am about to start restoring it. I took a few pics for ya'l to help me out because i'm about to buy a rear bumper for it and wanted to know if all i need is the mounting brackets and the bumper. It looks like it was cut before so i need to know exactly what i'm gona need.

Also my bed sags on the rear of the driver side. The front of the bed looks straight its just the driver side rear corner. Has this happened to any of your trucks if so what was it? I was just curious and thought I might as well ask before i start getting into it. Any help is appreciated thanks.