There is a very specific reason that you can't find any decent sugestion on these comments getting 5+. To put it simply, there's no silver bullet.
(for brevity I'll summarize my sugestion here, just combine the following: Control the PC with Remote Desktop for Android; Stream Audio+Video with Microsoft Wireless Display Adapter or a cheap Miracast Dongle; Have a PC that enables flawless screen casting over wifi, preferably using Intel WIDI on Win8+ and an Intel GPU; Have some very good domestic WIFI network; Forget long cables exist: they won't give you control over the PC, they are more expensive than a good enough router and they are cumbersome when you don't need their lenght anymore)
Everyone has very specific needs (marked with * are yours):
- some already have a gigantic library, so they need fast/reliable access to it wherever it is (also varies)
- most (of the afforementioned) formats are picky: lossless/lossy/weird audio formats and won't play everywhere nicely
- some just want to stream popcorntime (or youtube, or hulu, or [insert your brand of constantly updated content])
- others just want to cast their PC screen *
- then they realize they also want to control it from afar...*
But then, everyone has very specific, sometimes catch-all habits:
- some want to use the thing everyday, so they want swift performance, intuitive UI control and quick pinpoint of programing, even some form of "zapping"
- some watch sporadically and can take the hit on performance, usability, and even availability
- some want to use it once a year, but they want it to be flawless that once
Everyone has a certain flavour of a crucial part of the "system":
- no decent internet uplink from the ISP
- no decent WIFI for streaming HD content (remote or local)
- no chance to wire the place around for solving previous issue
- Linux, Mac, Windows, Android, HDMI (and its versions), DVI (and its versions), DisplayPort (you knwo how it goes), D-SUB..... USB (...Micro; Type-C lel)...
And then there's the real nitpicky shyte: low power usage, low noise, below 50$, must use https/proxy/VPN yada yada. Seriously I think there isn't two people in the world that, whatever solution they attempt, both of them will never be completely happy with that solution. But there's certainly one for your needs and I have a sugestion that just might be it:
Since you want video, audio, and control over two separate floors, you surely need to use that WIFI network. No way around it: and HDMI cable won't totally control your PC from afar. So you want at the very least 802.11n everywhere (router, tv-side, pc-side, smartphone-side). If you are thinking of streaming to pc from the interwebs, you might even need either wired connection router->PC, or alternatively dual band "ab" stuff so local throughput doesn't get chopped by the big mesh dynamics going on. Since we got the "medium" out of the way, now we need your "control device". Three words: Android Remote Mouse (google it). Now the most important part: how to get your display+audio out of that PC and into that HDMI? Unfortunately there's only one decent choice in my humble opinion (it has many names though...) Miracast/Widi/screencasting. And that narrows your choices a lot: PC side you need an intel CPU and Windows 8+ as they are the (to my knowledge) the most viable way to share desktops SEAMLESSLY; TV side, I will say you want a Microsoft Wireless Display Adapter or any decent Miracast dongle that suits your pocket. People say chromecast will also allow it but I wouldn't trust 2yo product on a constantly feature-changing (sometimes without warning) software platform.