I have a similar setup using Truenas to store data. I’ve setup a VM in Truenas that can access the data via NFS (easier to setup on Linux than SMB).

It’s nice to keep all your services contained in one machine, as long as it has enough resources, and will probably consume less power than running another PC.

I use qbittorrent, most people seem to agree it performs better than Transmission. It’s accessible from a web interface.