Where is the right place to post questions pertaining to building computer systems and related hardware components?
a shopping or buying recommendation
Of course, if you mean building as in building from transistors and diodes, then you probably want Electronics, but I don't think that's what you're talking about.
Super User is our site for computer hardware and software, so it should be a perfect fit there. A cursory search yields thousands of results blindly, most of which do appear to deal with building the machine (rather than, say, code builds or summat). The closed questions are closed not for being off-topic, but for other reasons. So as long as you have a solid question to ask on your hardware, you'll be fine at Super User. ♪