There is no need fire up a dedicated machine to do this. Use your router/ap running openwrt and connect a hdd via usb. The machine needs at least 128 Mb RAM (256 mb would be better). Install the transmission package, set it up, add a gig of swap space on the hdd and you are good to go. The AP runs 24/7 anyways so there will be very few extra power consumption.
Vpns often don't allow port forwarding (mullvad has stopped support recently if I remember correctly). You can just be a passive node and not often ports, that should work good enough.
Consider seeding parts of sci-hub. it's a project worth supporting imho.
You can just download once of the parts below with less than 12 seeds and set it to host without ratio: