Welcome D89 !! I also have started on a ROTJ build. Be sure to search for all the build threads you can, you will learn something different on each one that will help you on your build.
Do not buy anything off of ebay. Ebay items usually will not have the quality, size, or colors that are accurate for your build. I've made the same mistake before searching the site and learning differently. The members that make and sell items on the site will be your best source for accurate items. As far as helmets and armor goes, Wasted Fett makes the FPH2 ROTJ helmet and armor. RKD (managed by Villian Works) makes 2 sizes of ROTJ armor, Artakha sells a nice looking helmet also...
Below is a few good threads to start with, they cover a lot of information and show some of the process. Some of the links listed in the threads do not work anymore. When the TDH site was upgraded, all the old links quit working. So, to find them, you'll need to be creative and search the titles to find them again or search the author and dig through their postings to find the particular thread that you are searching for.
Anyway, the links below should get you give you an idea of how "Fett Mountain" is made, be sure to take your time, feel free to ask plenty of questions, the super knowledgeable folks on the site will be glad to help get you headed in the right direction if needed... Hope this helps you out....
Jayvees ROTJ's lil chunk of ROTJ love thread is excellent for a helmet paint up -
*UPDATE 2013* WIP : Jayvee's 'lil' chunk of ROTJ love' paint up
Zero's ROTJ Helmet Paint Up -
Zero's ROTJ helmet paint up
8r's ROTJ work in progress -
dv8r’s Boba Fett ROTJ Work in Progress