Not sure what is the answer for the first question, since my experience were limited to manage small-medium traffic relay (20+ GB/day). Maybe top relay operators will share their experiences later. I think top relays have big traffic more than 1TB/day.
> Hands on is sysadmin
Basic skills to manage linux or unix server such as configuring webserver (nginx, caddy, etc), setup proper firewall, ssh and account security, manage running service/apps (systemd, sysinit, etc), and basic linux/unix command knowledge were needed.
> Costs
Costs for running small-medium relay around $10/month . Varies depend on server provider.
> Preferred relay software
Currently using strfry. Other popular choices are nostr-rs-relay, nostream.