Buy the cheapest :- ) When it's for node, it doesn't matter. When it's for workstation, it does... and SuperMicro has a lot of versions, so if you find one, search it on their website to see if it has what you need (USB 3.0 slot, enough PCI slots,etc..).
Regarding memory, always buy the cheapest :- ) If you buy Supermicro, or Asus Z9PA (opposed to PE WS, first is server board, second is workstation board), you will need DDR3-ECC (1333-1600 Mhz), just find the cheapest. Remember, to run these in quad channel, you need 4 modules for each CPU. But it doesn't matter too much, because for rendering it doesn't matter a lot. Just don't buy 2x16 if you want 32gb :- ) buy 4x8 to run dual-channel, and if you need later 64gb, you will just buy another 4x8.