When you say "multiboxing", you're referring to running video games across multiple VMs, right? Only asking because most modern games take some pretty serious GPU power to run a single instance.
As for the box I'm planning on selling, it's a single socket X99 mobo with an E5-2695 v3 (14 cores | 28 threads) and uses between 64GB and 128GB of DDR4 ECC RAM. It would be a bit more expensive than what you were planning though.
My ultimate recommendation for you is:
A - If you aren't as concerned with age/core-count, I'd just buy a super cheap HP Z800 with a couple of X5690 CPUs for cheap on eBay
B - If you want to stick with slightly newer tech with better processors, I'd stick with doing a custom build as you planned, plus...
- Maybe change your CPUs to E5-2650 to save on power & money, but still retain core-count. Here's a great link for the processors -->
Intel E5-2650 v1 SR0KQ 2Ghz 8 core C2 VMware OK 95W Socket LGA 2011 Xeon CPU(S2) | eBay
- You could save some money & power on ECC DDR3 RAM by using 1333MHz DIMMs instead of 1600MHz DIMMs
- The chassis you picked is great as long as you MAKE SURE the board you buy fits ATX or E-ATX form-factor
- The power supply you picked is great. Stick with semi-modular power-supplies since they're cheaper than full-modular plus gives you options
- The heatsinks you picked are great, but you could probably get away with something cheaper unless you plan run your CPUs at 100%
- The SSDs you linked are good, but here are some 256GB SSDs that are almost the same price -->
- SAMSUNG hp 256GB SSD, 761884-001 -mz7pd256hcgm-000h7-opal 2.0- | eBay
C - If you want to "go big", I can sell you a fully built, eat-that-workload-for-breakfast, destroyer of a workstation. This would definitely be more expensive, but all the components would be more recent (Haswell EP/EX, DDR4, NVMe, Etc.). I recommend this if you are crazy.
Long story short, if you are fine with older components then you should pick
A, as you would likely spend between $400 and $500. If you prefer slightly newer components with more power and flexibility for your setup, I'd pick
B (which I recommend), which will likely cost between $600 and $800. Lastly, if you are crazy you can pick
C.
If you want any help picking components or finding good prices, let me know!