I'm looking to upgrade a Lenovo ThinkServer RD630 with PCI express hard drives for my Linux OS (one to boot from, and then a backup clone). The goal here is primarily speed, and also to free up hard disk bays on the main RAID array for other uses. I have two available PCIe 3.0 x8 slots on risers. The server has no extra SATA ports for internal drives, otherwise I would just do that.

I've been looking on Amazon for these drives, but I'm at a loss to determine which is the best. They range in price from a couple hundred to almost $5000. This is just for the OS, so I don't need much disk space, no more than 500 GB. I would like as much speed as I can get though, at a reasonable price. Any recommendations? Anything I should consider?