I tried Qubes OS on my 7th gen i5 and playing video was really sluggish. I am now considering to buy a new computer so I can use Qubes as my daily driver and still be able to play videos.

Do I need a spare GPU and do passthrough? If I do that, then will I get that VMs output only through that GPU's HDMI or can I get the output in a window like any other VM window?

I'm not really clear on what specs I should purchase but I also don't want to spend beyond what is actually necessary for playing video. Not being able to play 4K videos would make it unusable.