If you are interested in the 256GB Samsung SM951, try searching for MZHPV256HDGL-00000 on ebay or google. I managed to get one on ebay for £60.
Regarding NVMe, I believe, (if wrong then someone please chip in), that the NVMe version will only boot on a Z640 machine and will only work as a non-bootable drive on the Z620. Generally, only the newer motherboards support NVMe natively.