Maya is not a standard at all
You can use any software you like. I’ve tested pipeline with Maya, 3dsMax and MODO - clean and trouble-free workflow.
There is 2 elements of nice picture: textures AND material. Textures are images you want to use in your material.
Popular software for texture editing nowadays is Substance Designer/ Painter, dDo, Adobe Photoshop, Gimp(free). Also a lot of textures you will get from baking details from high resolution model(Which often made in your modeling problem or sculpting software like Zbrush or Mudbox) into low poly model.
Material is the way to combine all your textures into final, well, material
Rigging is the part you have to do in your modeling software. You can animate in UE4, but it’s complimentary tool, rather than main animation tool. You can enhance your animations, improve their blending, customize them and modify to get outstanding result(IMHO animation tools are VERY powerful in UE4), but base animations you have to create in modeling software
Wind + Foliage actually is shader magic inside above mentioned materials
For everything else - yep, Cascade is what you’re looking for!
You can use SpeedTree for modeling and animating foliage(no rigging needed). For humans and animals there are some special software, but they produce very generic and bad(at my taste) result so I recommend you to not rely of them. However it’s only my opinion of course and it’s okay if you choose to use it
It’s a tough question. In order to get really neat clothing you either sculpt it manually or use something like Marvellous Designer, which is outstanding tool(and weird when you open it for the first time :D) and produce realistic result.