Heres how I'd do it. Make these guys the hardcore veterans, and make the lighter "Chaos Raiders" or pirates or whatever as an elite choice in your Lost and the Damned codex. Lost and the Damned always fit the newer breakaways better anyway. Lost and the Damned just sounds like post heresy traitor marines like the Red Corsairs.

So squads are 5-10 everyone is a hardened marine veteran as chosen are now and make chosen something hideous. Marines can take one of five marks as people forget Undivided is a mark not the lack thereof. If anything its the superior mark.

Basically, Chaos Space Marines are naturally space marine sergeant stats (which is still in the realm of decades to centuries of service). Chosen are going to be able to take gear that will out shoot Sternguard or outfight Vanguard.

Basically you'll have six configurations for units: Unmarked, Undivided, Khorne, Nurgle, Slaanesh, Tzeentch. Although I think you may be better off splitting them into six choices for different slots. Ie. Beserkers, Plague Marines, Rubric Marines, "Forsworn" (Undivided), Noise Marines, Chaos Space Marines as your six troop choices (mein got) because more units actually makes the army more streamlined. You can tool your weapon choices to the unit rather then to the army. Marks are clumsy. Its not that hard to think of names for Slaanesh Havocs vs Khorne Havocs. One could have heavy flamers, heavy bolters, and multimeltas; the other could be done with sonic weapon options. Regardless I'd make Havocs like Longfangs rather then devastators. Less is more. Then you could have special flamery things that shoot plague gas, or plague launchers that either shoot acid for anti tank or plague gas for anti personnel or just plain old heavy bolters for Nurgle Havocs. Then you can customize an assault unit to each choice for your fast attack slot. Khorne and unmarked might be jump infantry. Nurgle might be mounted in a rhino and armed to the teeth with plague claws (I'm kidding about the name.)

Black Legion: Assorted Marks/Undivided
Word Bearers: Undivided
Night Lords: Unmarked
Iron Warriors: Unmarked
Emperor's Children: Slaanesh
Thousand Sons: Tzeentch
Death Guard: Nurgle
World Eaters: Khorne
Alpha Legion: Unmarked

That's six totally different styles. Unmarked will still have options like infiltrate and scout and stuff on their units. Khorne will have rage, counter attack, and furious charge to represent their battle prowess. Nurgle will have Feel No Pain and Relentless etc. I'd also change the possessed unit to a possessed model like the Lone Wolf. Think a weaker version daemon prince type. Shy of a champion possessed with a powerful daemon I don't think possession is anything that out of the ordinary. IMO, most chosen would be so saturated with chaos you'd think they were possessed. Thats what being chosen is. You're as much the chosen favorite of the dark gods as the chosen of the warlord.