Bandwidth, no data, but it's likely low. Email is a low-bandwidth application. There are about 700 subscribers.
The entire Mailman installation, including the contents of the archive, is small, about 200 MB. Most of that is the archive.
You could probably host the whole thing on a Raspberry Pi with a USB stick, but I wouldn't want to.
When you need hosting for that kind of service we can offer a VM on our VMware ESXi 6.0 host in Amsterdam (on 44.137.42.0/27, BGP routed via 44.137.0.0/16). There are 2 HPe Proliant servers each with local RAID-1 disks, a VM image is copied from one host to the other nightly and we can change over manually on catastrophic hardware failure. Offsite backups of the VM images are made as well. It is connected to Internet at 1 Gbit/s and to our radio network at 40 Mbit/s. Network access is via a MikroTik CCR router providing firewall configurable at port level.
This is where gw-44-137 runs.
Rob