Custom SQL Server Computer Build
This is essentially a hardware capacity planning question. The problem with them is that if you give enough details that we can tell you what hardware you need, it'll become "too localized" to your particular set of circumstances. That said, we've got a number of pseudo-answers already written to help you:

Can you help me with my capacity planning?
How do you do Load Testing and Capacity Planning for Databases
How do you do Load Testing and Capacity Planning for Web Sites

Those are from Server Fault's list of Canonical Question/Answers.

They're not all directly related to what you're asking for, but tangentially similar. It boils down to the fact that this is your job; if you want to get it right you really either need to learn how to do it, or find a consultant who does.

One last thing; the TCO of building your own server from components is always higher on the average than buying a prebuilt server from HP or Dell. I happen to prefer HP, but the SE Sites run on Dell and the site seems to work quite well. It will be cheaper up front to build your own; but warranty, compatibility, scalability, and support will all chew that cost savings like a hungry dog with a fresh steak. 

Edit:
Since you're discussing RAID levels in the comments above: What are the different widely used RAID levels and when should I consider them?