Just search for “kneeling pad” or “gardening knee pad” and you’ll find a bunch of pretty durable options for around $10. Or, hell, just get a pair of skateboarding knee pads. You might not be the pinnacle of fashion, but your knees will be well protected.
I don’t know if I’m alone on this, but I just bought the biggest 5400rpm HDD that was in my price range when I set up. Might notice the slower speed when doing a big data dump, but for streaming purposes you can run many 4k streams concurrently and the bottleneck would probably be your network speed before you hit a drive read bottleneck.