If a 501st approved suit at that level is your goal , the first step ( in my opinion) is to sign up with your local 501st detachment. Mine has a cadet/advisor program but I'm not sure if they all do. There you can get direct advice on what will be expected of your build.
Most people here build their suits from a variety of different vendors - each one offering a piece they specialize in. There are vendors that offer complete suits ( BobaMaker being one, I think RS props does as well) but they will take a long time to commission due to the amount of work Involved. Check out the various build threads here for ROTJ and you should be able to find some great information.