I would actually buy somethign pre-built.
Here is a nice sample System that includes an Keyboard, Mouse, OS, and Monitor.http://configure.us.dell.com/dellstore/config.aspx?oc=bv1cs3x2&c=us&l=en&s=bsd&cs=04
(All Items missing from "Tom's builds"
Toss in Next Day on-site service for a year all for $439 its a really good price.
And if you look around, you can likely get an even better deal.
It's rarely worth the time/effort to build General Duty Office machines.
Don't forget the extra time and effort by accounting to itemize proper depreciation on the individual parts.