Bare Metal Installation

Do you have or can borrow another Mac, if so login to the App Store using your AppleID you use on your iMac

and download El Capitan from the Purchased page. Once downloaded you can create a bootable USB by following the instructions here, How to create a bootable installer for macOS - Apple Support

Once you have done that plug the USB into your iMac, reboot holding down the option/ alt key, from the boot selector screen select the USB, flow the instructions and select your new internal SSD as the destination disk when asked to.

When booted off the external, can you download the installer from the App Store, and when it opens, go through the screens, then point it at the SSD?

Disconnect the external drive from your Mac, login to the App Store using your AppleID open Purchased and download El Capitan from the list.

Does your Macs internal drive not have an OS on it?

You are welcome.