Not to be offensive at all, but coming from a Pana HTIB, you can only go up. And I say that as a fan of Pana product in general.
Focus on speakers before sub or receiver. Disregarding setup/placement/tweaks, the speakers are by far the most important choice in regards to purchases.
Since you have so much wattage in your truck, you will have to make sure that either your speakers are easy to drive (which does not preclude you from buying good speakers), and/or you don't crank them so much that you cause damage to both receiver and speakers.
I vote for the best pair of speakers, for now. Perhaps receiver next. Then either sub or center speaker, depending on your thoughts. Surrounds last.