1. not at all, a small xmpp server is barely noticeable on server resources. Prosody alone should use less then 50mb RAM in your proposed usage scenario.
2. Yes, somewhat tricky but 100% compliance is not needed and Snikket really helps with getting it running well. Edit: my server has something like 95% compliance and works fine. There are a few things in the last 5% that are more tricky or undesirable (like open in-band registration).
3. Depends. There is a official debian/ubuntu repository for Prosody, so basic functionality would just update with everything else on your system. Snikket provides official Docker images, so that is also fairly easy to maintain.

Have a look at this article which compares some resource use. For small instances Prosody uses even less resources then ejabberd, but ejabberd scales better for larger servers.

Is it resource intensive for a small number of users?

No. Prosody is very resource friendly, especially for a small group of users. You can easily run it on a first generation Raspberry Pi.

Is it tricky getting it to comply 100% with the standards expected by compliance.conversations.im?

I think you just have to enable some modules in the config. I'm sure there's a guide on how to do it somewhere.

How much effort does it take to maintain it?

Less than Matrix-Synapse. You set it up once with all the modules you liked through the package manager of your OS and then you just auto update and forget it. Of course you should do regular backups of the data it stores, but apart from that you should be fine.