M.2 is the form factor. The important thing to look for is NVMe, which is PCIe. If it's not NVMe, then the M.2 drive is probably some slower technology like SATA. And yes, you want PCI 3.0. But I think 2.0 is still better than SATA. You can use a riser adapter card like the one purplecfh pointed to. There are also riser cards that will hold up to 4 of the NVMe sticks. If you go down that path, be sure to check the specs if you want to put them in RAID.