I realize this thread is a few months old, but just like Lockdown I went with a Lenovo Thinkserver TS140 and windows 8.1, and I know Server OS's and hardware quite well.
From an OS perspective you gain little using a Server OS like 2012 over 8.1 *unless* you need the built in server roles like DHCP, Active Directory, Hyper-V, etc. For simply accepting camera streams the OS makes little difference because the connection is working at a layer that windows really doesn't care about.
Windows XP made a rather clunky server compared to 2003 (god knows I supported many of them used as cheap point of sale SQL boxes), but modern Windows OS's are pretty much the same beasts with different feature sets.
Using Windows 8.1 actually has some advantages over Server 2012. The biggest one is it's cheaper. The next is you can use popular freeware software packages like Macrium Reflect and other imaging software and not get a nasty message about needing to buy the premium version for a Server OS. What's sad is many security companies around my area will only sell Server OS's and higher end hardware packages that really aren't required for security recording. They will also insist the customer buy CAL's, which technically aren't required in this case. If Microsoft didn't want you to use 8.1 in this role they would have slashed the connection limit.
Entry level servers like the TS140 are also great bargains for this role. SoHo Servers tend to be pretty tough and solid and my experience is they are darn near as tough as their rack and blade mount cousins. They lack all the superfluos crap that causes more feature rich desktops to fail. The TS140 can also be had for around $350 with 1225 Xeon, 4gig of RAM and a 500gig HD. The only problem I had with the TS140 is the Lenovo version of the RAID set didn't get along with 8.1 as a boot OS.
