Just a few things to think before doing the army from my side

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1. Never start a new army painting project with the general. He will be a centerpiece and should look the part so rather go with the humble chaos warrior first. They are easier to paint than the highly detailed character models.
2. Get a concept for the army. With chaos that means either painting everything in the favored colors of the godly marks each model wears (red khorne, green/yellowish nurgle, ...) or going for a kinda unmarked style with one color scheme across the whole army. My chaos lord favors orangish red and forces everybody in his entourage to wear that even if they fav or nurgle. This decision is really important since it really decides how you will paint the models.
3. Decide on basing... There is lots of basing you need to build up before you attach the model to the base.
That's all that comes to mind at this second but you should really plan out the project a little before jumping in since you will paint many models in the chosen scheme. Usually many more than with
40k marines.