Pulling Data from an External RESTful API Service to Populate Existing Fields?

What exactly is your question? if is the topic title, then yes, is possible. The list you made are all custom stuff. You can read about Sugar’s Webservice here https://support.sugarcrm.com/Documentation/Sugar_Developer/Sugar_Developer_Guide_6.5/Application_Framework/Web_Services/index.html

best regards

I think specifically I was asking, how reasonable would the development for this be?

Is it relatively simple for suitecrm to GET data from an external API, and then populate existing default fields?