Like it was already suggested, everything since intel 7th gen with quick sync should do the job for transcoding 4k hdr 10 bit releases, even the low tier i3 ones. You will also not need much ram for transcoding 8 should be fine, with a larger raid array go for 16 or above.
When you watch stuff just once anyway, honestly you will not need much, a couple of TB should be more than enough.
Not aware of any service that does automatic downloads based on a queue.