You ask if any parts are tricky, and I would say that getting all the details just right on his pants can be tricky, but not hard. You just have to plan what goes down where and when they overlap and all that.
For the scarf, I used fleece because it held itself up really nice (I have really quick tutorial if you need it for the scarf).
I wouldn't use felt for the trim. It's a uniform, felt would cheapen the look. For the red, I used extra wide double fold bias tape. It's really easy to go around the edges and stitch down. It also makes it much easier to make sure it's all straight and uniform in size. As for the fabric, I used a polyester gabardine for my Lavi, my friend's Lenalee, and an Allen commission.
Don't forget the beaded parts that hang from the buttons to the E button on the jacket. Those are very prominent on the uniform.
Here's how mine turned out
(along with pictures of my friend as Lenalee).
Feel free to ask any other questions if you need a little help. I love the 3rd uniforms and have had fun making three versions of them so far.