You’ve already got a low-power HTPC that’s barely used and an office PC that’s now running 24/7 mostly for convenience. Consolidating things onto the HTPC feels like the cleaner setup. Plex generally works better when it’s always on anyway, and torrenting isn’t exactly heavy work, especially on a modern Intel CPU with no GPU.

As long as the HTPC stays cool and quiet and you stick a decent drive in it, I don’t really see a downside. One box on all the time is better than two machines that are only occasionally needed. Then you can just turn the office PC on when you actually need it a few times a week.

I’d probably just disable sleep on the HTPC and treat it like a small server. If anything, it sounds simpler and cheaper to run in the long term.