I made 1 of Brian's 3 slat side chairs in a class I took with him at Kelly Mehlers School of Woodworking. The process was a bit complicated but doable. Now that Brian's in Asheville I'd highly recommend taking a greenwood chairmaking class with him. After taking a class and building 1 chair it'll be much easier for you to build another. I'm not really a "greenwood" chairmaker but the class taught me ton on chairmaking in general. I also would highly recommend Jeff Millers book on Chairmaking.
As for for the shavehorse. I used one of the schools but I remember Brian said he published plans in an old Fine Woodworking of the one he built which I believe is very close to the 1 lie nielsen sells.
Sooo.. I guess my answer is take a class 1st. Chairs are pretty hard and it'll help give you confidence so you'll want to actually build more. Not to mention Brian is a REALLY good teacher.
Over the past 3 years I've made 4 chairs and a stool. I started out with John Alexanders DVD "Make a Chair from a Tree". I watched the DVD several times and referred to it often while making the chair. The tools are simple and easy to acquire. It took about 3 months to complete the first chair. Spent a lot of time drying green lumber. The next 2 went a lot faster as I had dried enough lumber for all 3 chairs when making the first. The first chair was built following John's methods exactly. The chair is more comfortable than it looks, extremely strong and only weighs 5.5 lbs. The next 2 I started incorporating power tools. Saves time and elbow grease.
I've had a shaving horse for several years made from a 12 foot 2"x10". If you are going to start making chairs I'd build a shaving horse first. I really like the Brian Boggs design and plan to build one in the future. I've figured out how to make one from a single sheet of 3/4" plywood by laminating pieces together. I can email you a PDF of the layout of all the pieces from a 4'x8' sheet of plywood if you like. Remember I haven't actually built it yet so some of the dimensions may be off. I used the article from FFW to get the dimensions.
You can find pictures of some of my chairs and steam box (something else you're going to want to make) in my gallery.
I have limited experience in chair building but I'll try to answer any questions you have.