Case may not be important to you; however, it is typically important for HTPC. If you think you will want a good case that will look at home in a stereo setup, consider getting such a case with your initial build.
Also, you did not mention television recording (DVR) as a function you seek in your HTPC. It is a very useful feature of HTPC for many...something you may also consider.
Your costs may increase a bit, as you have identified no input device nor operating system.
A.L.a.E.o.t.U.S., as proven 3/21 - never forget.
Knowledge isn't Truth; it's just mindless agreement.