There are two ideal camps that most people will fall into re: building HTPCs in 2020.
1. Videophiles looking for the absolute best video quality
2. Normal people that want something that works, looks pretty good, and is economical
If you are in camp 1, the basis of your build will be the most expensive GPU you can afford in conjunction with madVR (which involves scripting, detailed understanding of upsampling algorithms, etc.). Cost: ~$1000. Power usage: 300+ watts. Quality: Excellent. Most difficult.
If you are in camp 2, you should try really hard to eliminate the HTPC and stream to your TV using built-in apps (Plex, etc) or an embedded device (Roku, Fire stick, chromecast, etc) and use a NAS or PC functioning as a media server with DLNA or some other protocol. Cost: free or ~$40 for a DLNA client. Power usage: 0-10w. Quality: Acceptable. Easiest.
If you fall somewhere in between the two camps then the price/performance ratio is going to go downhill but it's still an option If you want to run Red October HQ (which is basically the middle-ground between the camps--easy to set up, looks really dang good, just requires a more powerful GPU than a typical desktop) then you will want to base your build around a GPU that is on par or equal to a GTX 1050 or thereabouts. Cost: ~$600. Power usage: 200+ watts. Quality: Great.
If you want more control of the client (say you want to run MC Theater View) then follow the Raspberry Pi 4B video progress over in the MC for Linux board. It's almost possible to build your own SBC HTPC running MC. Cost: ~$50. Power usage: 10w. Quality: Good.
Once the Pi ARM build shapes up, you could build a HTPC around a RockPro64 with or without an external low-profile GPU. Cost: $100-$300. Power usage: 10-100w. Quality: Very good to great.
On the horizon are the Ryzen 4000 series APUs. These will very likely be good enough to run ROHQ out of the box but that won't be known until they can be benchmarked. At the very least they will accommodate madVR with custom scaling. Cost: ~$400. Power usage: 70W. Quality: Very good to great.
You can look into an existing embedded ryzen board like the Udoo Bolt. Cost: ~$500. Power usage: 25W. Quality: Good to great.