I can't really answer how much maintenance would be required, as that really depends on how involved you want to be, whether you operate the hardware, if you moderate, etc. There is plenty of different relay software to choose from, with different capabilities. Easiest way to find them would be to search 'nostr relays' on Github afaik. There are also hosted/relay rental type services, like relay.tools that probably offer a lot of the functionality you might want.