Without knowing what iMac model you have, I am going to have to guess 14.2...
I like Western Digital Black drives, best place to buy is NewEgg...
https://www.newegg.com/black-wd6003fzbx-6tb/p/234-000G-000W5
Make sure you buy from NewEgg, watch the seller carefully...
Careful, some of the iMacs require a Thermal sensor, the original drive had one, the replacement does not.
https://eshop.macsales.com/item/OWC/DIDIMACHDD12/
But if your going to have the iMac open, I would add memory, and consider a new SSD blade drive...
You probably have two 4 gigabytes chips, the 14.2 supports 8 gigabyte chips, and has four slots, two should be open...
https://eshop.macsales.com/shop/ssd/owc/imac-27-inch/2013-2019
Some of the other models of iMacs have memory soldered to the board, some have only two slots, careful...
You probably have a 128K blade SSD, a new blade can be found here...
https://eshop.macsales.com/shop/ssd/owc/imac-27-inch/2013-2019
If you replace the hard drive with an SSD, then buy a 2.5 to 3.5 adapter, it will make life much easier...
https://eshop.macsales.com/item/NewerTech/ADPTADRV/
The 860 EVO from NewEgg are known to work...
https://www.newegg.com/samsung-860-evo-series-1tb/p/N82E16820147673
I would not reinstall fusion, as the drives are not longer Apple, and problems may occur.
I would install the OS to the blade drive, then the programs and data to the hard drive....