There’s a lot of good stuff here: How to Create Realistic Trees : Blender Tutorial - YouTube for modeling a tree by hand in Blender. You can obviously skip over the material parts because you will be using UE4’s material blueprints, not Blender’s.
For a forest, you probably don’t want all of your rendering quite that detailed. So what you can do is take the branch part of the video and just create planes or similar flat low poly shapes and apply a full branch texture to them. This won’t be as heavy to render as individual leaves.
I am still working on animation, so I don’t have the best answer. However, if you download the Content Examples project (you can get it in the launcher) there is a map that shows off vertex painting features and at the end of the line is a complex wind animated tree. I haven’t quite had the to reverse engineer that yet.
The above youtube tutorial uses a bit of vertex painting or weight painting so if you watch it you should basically have an idea of how to do it. You’ll want to paint your vertexes in Blender to be used later for wind in UE4. For example, you could paint the ends of your branches red and have them fade into pink as they go further in and then pure white at the base of the tree. You can use that in UE4 to have the tips of the branches sway in the wind more than the center of the tree and the base should not move at all.