The slaves to darkness start collecting is actually purely full of models you can’t get outside the bundle. Sure, you can get chaos warriors and knights on their own, but they’re different, older kits. The kits in the bundle are relatively new push-fit models not for sale elsewhere by GW.
The wife and I got fairly lucky starting out with khorne/slaanesh cause we were able to pick up 2 copies of the wrath and rapture box set really cheap. For any summoning armies I really recommend trying to find these value boxes, at least here in the uk the shadow and pain box is still available online for about £85 and it makes a very good start for slaanesh. The mad thing about the box is if you can be bothered to piece out the daughters of khaine stuff on ebay you get about £70 back from them too so about £15 for around 700 points, pretty good deal
Quick note on Tzeentch: You dont HAVE to have double the blues as you have pink horrors. I have 2 units of pinks in my army and I only have 3 boxes of blues (instead of 4) becuase it's so unlikely that all pinks will be dead at the same time, without any of the blues being dead
Quick thoughts on the Legion Of Azgorh? I knoe that they have basically no rules and only a small PDF, but what's your opinion on them? I personally love those stocky, evil bastards