How to create a drop-down block?
I think the Ajax Docs gives a pretty good example. Here are the pseudo steps:


You render your view block with 5 elements with "read more" submit as a form. The wrapper is is "my-ajax-block".
Clicking on the submit will create an ajax call (with help of built in ajax mechanism of Drupal).
You will make a call for all the items you need (50 pieces in your case), hopefully with the same service you provide the 5 one.
And you just send back the ajax response which will override the "my-ajax-block" with the fetched content.