iMac with dead internal ssd

It's likely that your model just has a blade SSD installed on the back side of the main logic board. If that's the case, it may not have the SATA cable installed, in which case the kit that you mentioned wouldn't contain everything you'd need (you'd also need to purchase an iMac 27-inch Late 2015 SATA cable if you're planning to install a 2.5-inch drive). Alternatively you could just replace the blade SSD on the back of the logic board (you'll want to remove the failed one anyway to prevent it from causing any weird behavior). Here's a link to the blade SSD replacement guide, and here's a link to the SATA cable replacement guide. Good luck!