Depending on your specs, I don’t think you need to buy hardware. You can scale later if you run out of resources. This is how I’d separate your stuff:

• Desktop: SteamOS (or Bazzite IMO), for gaming. Install Kodi on it for the HTPC interface, and add a shortcut to it on the big picture UI
• RPi: Home Assistant (they have images for the Pi, which will let you install updates and addons from the gui)
• Compute stick: Some server OS (I personally like Fedora Server, but loads of options are available here. Bazzite is Fedora based, so you can use knowledge you gain on one, on the other). Nextcloud, your *arrs, torrent client etc. Stuff that is okay if it goes down for a bit.
• Laptop: Same OS as the compute stick. Whatever doesn’t fit the stick, plus Pihole/Adguard. For DNS though, I’d recommend using a cloud provider instead of self hosted, because if it goes down, you’ll be stuck reconfiguring your DNS before fixing it (since if dns goes down, you can’t easily access websites to look up stuff). IMO NextDNS is a good option, both free and paid tiers, and is does adblock lists too.