Replacing hard drive with SSD or PCIE

I have been wanting to replace my SATA hard drive for a while with an SSD for awhile, but didn't want to open up my iMac. My power supply has been failing and I am now forced to replace it. While repairing this I want to replace my Fusion Drive with a SSD. I initially was going to buy the SSD upgrade kit, but then read online it would be even better to replace the PCIE SSD. Is this the case. If I were to replace this, can I remove both the PCIE 32gb flash drive and the 1tb SATA drive and just replace with a 1tb SSD PCIE? Is there an issue with the fan temp? I don't need the extra storage of the SATA. Thanks.