Wrong CPU. The 2637v4 is a quad core 3.5GHz-turbo-to-3.7 with 15MB cache that's designed for dual-socket operation and costs around $1000.
The 1620v4 is almost the same CPU except it turbos to 3.8 and only has 10MB cache, costs a third as much ($294). The 1650v4 is actually a sweet CPU, six cores 3.6GHz-turbo-to-4.0, around $600.
The one downside to the E5-16xx CPU's is that they do not support LRDIMM. This probably isn't a big deal.
Having built something like this but with HDD's, I'll also note that I had a hard time getting the CPU to do much more than yawn even under fairly heavy load. My *impression* is that the 1620 would be fine, but once you're paying for all the other components, a few hundred extra for more cores and the slight speed bump is really nothing.