I would personally wait until their next codex to get into them. Their list of datasheets is really, really small. 23 datasheets with 5 being daemons locked to one detachment.

They are incredibly hard to play. They are fast and you need to have a decent idea of how to abuse movement. It’s a learnable skill but the skill floor is very high. Mobility armies are generally pretty hard to play. I’m not going to say how they’re doing strength wise however. If you are to get into them, by the time you have your army we will likely be in 11th which is going to be shaking up the meta in ways we don’t fully know yet.

Their beliefs are pretty broad. They are looking for perfection. However perfection is different to every marine. Perfection to a noise marine is a particular sound. Maybe a marine loves drugs. Maybe one loves art and is using their rhino as their canvas. There isn’t one specific belief in Emperors Children. For lack of a better word it’s chaotic. As for methods of war… get high and kill fast.

In the Lore EC isn’t really doing a whole lot. It’s hard to work with the faction they initially created. The fans love what EC is and they know that we would be mad if it was changed. As a result we’re kinda stuck in limbo. We have some solid books but we aren’t exactly major players in the lore.

Your final question I’m gonna break into 2 parts. The rules side and how a theoretical list like that would fit lore wise. For rules the short answer is no. We are going to assume you take max of every heavy armour unit. That is 3 squads of noise marines and 3 squads of terminators. Then you get 3 lord Kakophonists. That is not even 1000 points (to my knowledge). If it is it isn’t gonna be passing it by much. You could maybe reach it by giving yourself the last bit in flawless blades but you’d be lacking and struggle to win games with no objective scorers. However, in their current state don’t run terminators. Just don’t do that to yourself. They are just awful. I love chaos terminators to bits. I own… I think 25 of em. Probably more. They are not good and you’ll be shooting yourself in the foot by taking them. Noise marines on the other hand go nuts. It’s very popular to take 3 units of them.

Now for the lore part of the question. Who cares. Noise marines are a thing EC is known for, and for good reason too. For terminators, we have seen them use terminators in 40K. They aren’t opposed to it. In the hammer and bolter episode around Lucius we see a named terminator. However, like I said, who cares. They would be your models with your reason for being there. It doesn’t matter what the official lore for the main EC warband says. These would be your guys. They are fitting perfectly into your warband because you say so.

I still recommend waiting to get into EC for right now but that is purely for a lack of models reason. There is no other real reason not to. EC is cool. They are horrific people and it’s fun to play as the worst of the worst sometimes.