What to buy?

So im very new to the hobby. Custodes is my only army and was lucky enough to preorder a box of Champions. Of the product currently released, is there a buyers guide for what models are usable for EC? Like i know the CSM rhino works for them, but what should i be buying to help build out an army list?

Sorry if this has been asked before, just very hesitant to buy anything due to my lack of knowledge and where to look.

Edit: Side question, what should i be looking to build in general. Like with the EC box, infractors or terminators?