As a test about 6-7 months ago I set up a Zulip server on a VPS to test it out. Because it was a test I can't speak to scale, upgrading, or maintenance, but I would hope most any VPS would easily support ~30 users. The install process was unmemorable. I just had another look at the docs because I couldn't remember the process was. This is probably the best endorsement that can be given because the worst-case would be hunting down dependencies and having to compile stuff and tweak config files and it taking up most of a day or two. Instead, it was probably less than an hour from going to my vps' website to provision a new server to logging into Zulip from the client on my phone. I don't see any reason not to try yourself.

I'm not sure of your proficiency, but if this is a public-facing server you may want to either go with their paid service or hire someone to do the initial setup and walk you through maintenance since there's more to securing a server than just installing and getting it working.

Probably one of our users can better to attest to this than I can, but it's really easy for someone with that background to maintain. For installation, we've put a ton of effort over the years into making all the commands Just Work if you copy-paste them, adding custom error messages if you run something as root you shouldn't, etc.

And we put a ton of effort into making the upgrades, even for major releases like this, be a single command that Just Works for almost everyone.

I know this because I personally respond to every single report of issues installing/upgrading Zulip and have been making changes (even if the issue is basically user error) after most of them to help ensure they don't happen again for a few years now.

That said, if you don't want to deal with doing that work, Zulip Cloud is very reasonably priced for situations where all the users are employees (compared to payroll, it's effectively free) and for other use cases, we have steep discounts (e.g. free for open source, a huge discount for education, etc.).