iMac 27 2015 and msata conector?

Apple's SSD is a custom M.2 blade drive. So far no 3rd party suppliers are offering the needed SSD. The HD is using a standard 3.5" SATA drive connection. So you could replace the HD with a SSD. But I don't see that as the best direction here.

Here's the IFIXIT guide on whats needed to replace the SSD: iMac Intel 27" EMC 2639 SSD Replacement.

As you can see you'll need to take the system completely apart. I must warn you! You need the correct tools and take your time as you can damage the display taking it off. Which would make it a very bad day.

I would strongly recommend you get the model with the largest SSD you can afford, then later put in a HD if you still need deep storage.

Or, go with an external Thunderbolt RAID drive set which would gain better throughput than the built-in SATA HD drive.