One cannot read everything nor does reading something mean that one understands what one has read.
To make sure the reading that one does involves the best understanding it is useful to focus on introductions of these peripheral works, focusing on the selections of the originals referenced by the authors of these introductions.
In the case of the study of Augustine, Plotinus’s Enneads would be a peripheral work. Finding a contemporary introduction or commentary on the Enneads would be preferable to reading the Enneads without such guidance and risking misunderstanding of what one has read.
One work on Plotinus that might be worth considering is Dominic J. O’Meara’s Plotinus: An Introduction to the Enneads, Clarendon Press, Oxford 1995. His perspective attempts to make sense out of the Enneads based on a tradition encompassing Plato, Aristotle, the Stoics, the Epicureans, Augustine, Descartes and others. See his Epilogue for more information on the influence Plotinus had.
The book is short, only 119 pages, but it may require multiple readings. References to the Enneads can be read using the translation by Stephen MacKenna and B. S. Page available online at Sacred Texts.
According to O’Meara (p 113) Augustine's reading of Plotinus “opened the way for his conversion to Christianity and had at first a major impact on his thought. When he became a bishop and a very powerful Church leader, he sought to restrict more and more this Plotinian influence.” One way to approach Augustine would be to keep a question open in one's mind of why Augustine changed his mind about Neo-Platonism.