You are looking for a cost-effective way to add certain important functionalities to your fairly basic non-profit WordPress website. Like non-profits everywhere, you are constrained by budget and technical support and prefer solutions that are low cost (or free) which requires very little help to install or maintain.
Here are 16 WordPress plugins that may provide one or more of the capabilities you need. With these plugins, you can accept donations, add calendars and opt-in forms and improve website security, among other things. Take a look, there might be something here for you.