Unless they have an active IT department that is already hosting various websites for them, it is probably better to show them some hosted offer like this: masto.host

As for your questions:

1. Medium difficulty as far as self-hosting goes. Mastodon is a Ruby on Rails app.
2. Really depends on the number of users etc.
3. See no. 2. a few hundreds $ or so.
4. As others have mentioned, let them make an account on an existing server first, it can be migrated to your own server later.