Starting with Diogenes Laertius is a good way to get an overview, then reading DeWitt next would give an in-depth view. Then the two Ciceros; I might even read these before DeWitt.
The first I ever heard of Epicurus was when I read Cicero's On The Nature of the Gods. I was trying to come to grips with the Stoic idea of providence... after reading Vellius I haven't touched another Stoic text! So this and On Ends could come before DeWitt, and then when reading DeWitt one would have read most of the basics.
Reading Philebus before Lucretius seems interesting to me as it dovetails well with DeWitt's commentary and really points out the contrast between Epicurus and Plato. By this point one might be ready for Lucretius....
I really wish that I'd had a list like this to work through when I started exploring EP. I did use Elemental Epicureanism (and still do) and Hiram's "getting started" page, but with so little background in philosophy I had no sense of the scope of the texts or the relationships between them.