High-Five.
As others have helpfully suggested, buying a brand server/pc is easier and compatibility issues are often lessened, you miss out on the one good thing about IT: learning more about all things IT!
+1 build your own, its good experience and a proud moment when you realise you’ve built something cool AND useful!
Ive built several ESXi whiteboxes, and could not agree more.
Im using Adaptec 6405e RAID cards, they are around $250 - $275 aussie.
8 Core AMD Bulldozer
32GB DDR3 1600
4x 4GB WD greens (you really really DONT want to use greens if you can help it…Ill gladly give reasons 
I think its a gigabyte mobo, 970 something from memory.
This thing purrs very nicely with 11 VMs on it, usually 5 are running 24x7. includes win8, 8,.1, server 2012 standard, linux, win7, wordpress, myob and it also is the media server for the house!
NOTES:
NIC compatibility is critical, it wont boot without a driver being available You can add it to the installer ISO, ask around if you need to know how, its pretty easy, the hard part is getting the exact damn driver lol.
If need be, buy a couple of genuine intel nics, you dont need to use the onboard mobo nic/s.
Hardware RAID - do it, do it.
A UPS is useful too, for brownouts to not affect it.
Pick your own PSU, and for best power rating, get a “gold” one, 92% efficient apparently.
Have a beer / coffee
Theres some good experiences and advice here, I hope it all helps congeal into your one customized solution!