@Mara-Price That's a tough question... Personally I have bought about 1lb of core wool, because I know I'll use it and because it can be less expensive when you purchase more. For the bunny itself - it won't take that much wool. I wish I could give you more precise measurements.
The amount of colorful wool is hard to determine because sometimes you need to layer a lot, sometimes you need only a little of certain colors. Like I know I need a bunch of red, but I also am going to use a little blue, some yellow perhaps, but only small amounts of those.
I am going to be making my rabbit about 3-4 inches tall. So not very big, but it may be easier for you starting out to work a little larger so you can work in more details if you'd like! Small works great if you don't want to worry about details and just overall shape.
I'll be using the wire armature so that I can pose and bend my rabbit.
I'm going to link some shops I use in a moment!