You might want to sideboard some spells that destroy artifacts because one Pithing Needle can really throw this deck off. Also, other than for pure power, I don't see a use for Rakdos at all in this deck. It could be useful for its last ability, but I'd prefer Shadowborn Demon over it, since you have to deal damage to cast Rakdos if you don't have enough Apostles on the field, whereas Shadowborn Demon can be cast right away and help remove a pesky creature they control. You don't have to drop Rakdos, but maybe do away with at least one Xathrid Necromancer instead for it. That's just my personal preference.
Permalink
This would be awesome for Standard if Innistrad wasn't cycling out. I recommend putting Shadowborn Demon in, since he is a powerful creature for 5 mana as well as removal spell all in the same card. If you plan to use this outside of Standard, Harvester of Souls and Griselbrand are great for drawing the Apostles as well as Immortal Servitude, Blood Artist would leech your opponent's life away every time you sacrifice one of the Apostles, and Reaper from the Abyss has a Morbid effect that can destroy a non-Demon creature if a creature died this turn, which would obviously be the Apostle.
I think Centaur Healer is good as it is, but with Trostani on the field, Armada Wurm gives you more life due to it and the token, plus you can then populate the token. I would like to see this deck at FNM sometime before it cycles out.
Very interesting theme you got going. I assume the goal is to mill your "Enter the battlefield" creatures into the grave, and then bring them back from the dead to reap the benefits for a cheaper price. I think you could probably remove the Centaur Healers though, maybe for Armada Wurms.