To be completely fair, you don't actually "need" an M.2 slot at all. According to the age of the hardware you have fallbacks such as PCI to M.2 adapters that are blazingly fast.
If the hardware is old enough you won't even realize the potential of anything faster than a SATA SSD.
Ultimately, the question really comes down to how much time do you have?
"NEED" -- None. There is nothing in what you have provided to say that ANY M.2 slot is needed.
Are there benefits, even of SATA M.2 slots? Yes. Are there benefits of NVMe slots over SATA? Limited. Is any of this relevant? Unknown.