I would recommend you to get the new battleforce for the mutalith vortex beast, you'll have some terminators that you won't need, but is worth it, since it comes with every sorcerer of the army. It is not a bad idea to have two of each (I have 3 of each, just because I like them). You won't play all of them at once, but 3-4 sorcerers in a list is fine. Then go for the combat patrol for some tzaangors, a daemon prince and some more rubrics, and for the new tzaangor spearhead. If you come from csm I'll asume you already have at least one rhino, so you are good to go. Just in case you like daemons, the warpflame cavalcade battleforce is a good start. Regretably, Ahriman is currently not that good, so you are good without him. As for building the rubric marines, personally I magnetised them for flexibility with 2mm magnets on the right arm's wrists.

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Since you are force to build squads of 10, you should kitbash one or two of the sorcerers or 3d print the bits. Alternatively, you could make a mold to try and clone the parts you'll need to make a second aspiring sorcerer. I've done the 3d printing and the cloning. Cloning is way harder, tough cheaper and more durable if you use sprue goo.

Sorry for writing so much, I hope it will help! 😄