Yes, this is a lot to ask for
. I'm not a graphics artist, I stay as far away from models and textures as possible, so I'm afraid you're on your own.
As for turrets, you can edit the ship file yourself and add them - I'm pretty sure KillerWave's tutorials talk about that as well, and even provides you with links to the tools you need for this.
See, in the end, it comes down to this - unless you want to make a mod that only uses materials from straight from WCP/SO, you will not be able to just make missions and nothing else. You'll have to learn other aspects of WCP modding as well. It's not convenient, and it's not especially easy - but that's just the way it is. The editing community is very friendly and very helpful, but there's very, very few of us, and to make things worse, at this point most of us don't have time to do much. What's more, we all have different skills, so we're not able to help with just anything. I, for example, can help with missions and the like - but when it comes to graphics, converting ships into WCP and so on, I'm useless.