Well first off, you will need the Generals Handbook for up to date point costs and the new summoning rules. Also the Tzeentch battletome, obviously. The age of sigmar app is very good to have and you can buy the books digitally on there if you like.
Everything you have can be used in a Tzeentch army, so long as you give your marauders/warriors/knights MoT. Your blue & brim horror units will need to be larger though, while pinks you only want to run as 10-man units.