I just setup an old desktop that has a AMD Phenom II x4 955 that runs it just fine…that CPU was released January 2009 btw. So, you’ll be fine with an i5. Video card wise, nVidia GTX 1000 series or AMD RX 500 series cards. Their costs have come down a lot used
You don’t need a very powerful video card to play 4K content with HTPC. That only need to decode the video which is not too crazy. It is encoding that needs a lot of power and why for your server, having a good card is key.
You didn’t say what model i5 this is, but if it’s recent the built-in graphics would be enough. You can always buy a dedicated card later if it’s not.
Edit: Here’s someone doing a test of 4K on an 8th gen i5:
Kinda hard to tell the results since YouTube is recompressing the screen recording, though. But the general answer is: yes. And I wasn’t even referring to something this age.
The specs for that cpu does not list QuickSync so it likely does not have it. It’s benchmark score is 2200 so that is likely too low to handle decoding in software, but you can always try.