What should I look for when buying an SSD for an iMac? I have an iMac 5k, late 2015, with a Fusion Drive consisting of a 2TB hard disc and a 128GB SSD. I want to upgrade the SSD by buying a third-party one.

What specs should the third-party SSD have to fit in and work with the iMac? NVMe? PCIe? mSATA? M2?

There are two parts to this: it needs to have the right physical connector that fits into the Mac motherboard, and it needs to speak a protocol the Mac speaks. What are the requirements for each?

If I choose an SSD that speaks the SATA protocol, will it still work with my Mac, or must I get a PCIe one? I'm not worried about a slight difference in speed, since both are much faster than a spinning hard disc.
Your model iMac has a Serial ATA (6 Gb/s) connector for a 2.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD.

For best compatibility I suggest you purchase your SSD from a reputable retailer. For example, the following are some well-regarded sites:


Mac Sales 
Simply Mac
RAM City
Macfixit


By buying from a reputable store you'll be sure to get an SSD fit for purpose, regardless of manufacturer. Being reputable means they're only going to sell something that works and they'll stand behind the product they sell, regardless of the brand.

Also, many sites offer an easy interface to ensure you select the right SSD for your particular Mac.