The JD stuff you'd need would be a screen, GPS receiver, and ATU. Then of course the harnesses needed to connect all those together and to the batteries. 

You don't usually many used ATU (auto trac universal) but a new one is $1600 or so. Used screen like a 2630 is very easy to find. Just needs autotrac activation on it which almost all have by now. Receiver for the roof could be something like a SF3000 which are very common and cheap now. SF1 is the free signal, a step better than WAAS. All SF3000 and newer get that signal.