"I will be back"

[Axebringer]

I WILL BE BACK - said Arnold Schwarzenegger a lot of time ago. 
Well, now I'm back.

A lot of time ago I was trying to play WHFB. I loved the setting and the models. But I didn't like the rules, at all. I always thought that they were somehow clunky. Those were the times of the 6th and 7th edition. I played 40k instead with my Necrons.

But now, I was hearing so many positive things on AoS, even if at the beginning I didn't like how GW was handling one of their main games. But the rules are free, so why not to try those one day?

So, I still have models that I can use as Slaves to Darkness. Are they a solid choice for Slaves to Darkness army?
- Exalted Champion (two weapons, one is a model from Avatars of War range) x2
- Chaos Knights x5 (old ones with swords and axes, metal minis)
- Warriors of Chaos x15 (weapons and shields)
- Chaos Marauders x16 (flails and shields)
- Chaos Spawns x2
- 15x Chosen (great weapons, they are from Avatars of War range)

Second question, how the marks work? In WHFB I was running a Nurgle themed force but I'm painting again all of the models to be a mix of Undivided and Khorne. Are the marks working like in 6th and 7th edition (when the army's General has a Mark, all troops with different Marks than him are rarer in the army - Troops become Special choices and Special become Rare) and you pay extra points for them? 

I need to run a god-specific army (like Khorne Bloodbound) or can I (without any penalties) run a mixed force?  

Have a nice day, guys!
Happy wargaming