Sounds like an awesome idea. I don't think they should be actual chaos space marines though. I think a good story would be that the army was some warriors, led by a leader who was tainted by chaos, who overthrew the planetary government. They end up using power armour taken from the imperial marines they killed. Since they don't have the black carapiece it would explain the lower strength and toughness stat.
SOB, cannoness and celestians could all just be CSM like you said. I would maybe model most of them without helmets except cannoness and celestians. The idea for seraphim as raptors works, or maybe just use space marine assault troops that have had their armour defaced. Follows the whole salvaged space marine armour theme.
For inquisitors and priests I think you can use the normal inquisitor and priest models just make them chaos looking. The idea of modeling some religious fanatic who worships chaos might be a good idea.
Imperial storm troopers can just be converted guardsmen, again looking very chaosy.
In terms of vehicles the landrainder, chimera and rhino would be just the same. Immolator could be planetary vehicle converted to a weapons platform. The repressor used by the adeptus arbites would a good model to base it on. Same case for the exorcist. Although a possessed vehicle spewing death would be cool too (think hellcannon from fantasy).
Arcos can be modelled as demons or possessed, crazed humans under the control of a chaos worshiping priest. Same with penitent engines, if you go with possessed, crazed humans than maybe using imperial sentinels would be a good base for a model. Repentia could be modeled as crazed chaos cultist.
The imperial assassins can be modeled two ways. If you go with the corrupted inquisitor than most likely they were able to find some way to gain the services of an imperial assassins, thus you can use the models without converting anything. If you go with the fanatical priest who led the uprising to count as your inquisitor than
Culexus: can be just a human null who has been cybernetically altered.
Callidus: could be a shape shifting demon. It would be cool to have a model made specifically for each army your play against, haha.
Eversor: again human who has been trained and altered (maybe was a some kind of experiment of the leader).
Vindicare: not sure model wise
DCA can either be modeled by anything. I am sure some death cult assassins worship chaos or maybe they are the priest personal bodyguards.
