For flattening, you can do something very simple. Below is a pix of a variation on an exact width dado jig. With the opening a bit larger than the flattening bit, and side rails that hold your router fairly snug, but still able to slide freely, it will work for flattening. Pix of half inch shank bit is the type to use for flattening.
Been a very long time since I had anything to do with guitars, but I know that whatever the body is made of, you will need to make the neck of something like hard maple. The forces when you tune the strings will warp most softer woods.
The more I do, the less I accomplish.