I would buy an old Dell Optiplex with a SSF case like maybe a 790. I see these on EBay for $100 shipped, i5 cpu 8 gig mem, Win10 pro etc. Add a half height video card for between $25 to $75 like a Nividia GT 1030, 4TB hard drive $100 and something like a Flirk for a 10 foot interface using any old remote. If your receiver has HDMI you don't need a sound card as the video card carries the audio. Blu Ray burner $80. These are very quiet solid small compact cases and work great for HTPC. Forget about 4K for now and your all in for around $300.
"Smart enough to know better, to old to care" ------ Dedicated Home Theater, Mitsubishi HC7900DW Projector, 110" 16:9 Jamestown screen with variable power masking for CIW 2.50:1 to 16:9, Marantz 7009 with 7.1.4 Atmos with Ohm mains,3 DIY Subs (2 15" (1 ported, 1 sealed and a 12" 4th order bandpass), 1 DIY butt kicker, Custom Built HTPC, 10TB DroboFS NAS