The extra cost is just to cover the poly bushings over the rubber bushings. If you don't mind piecing a kit together, you can typically find poly at the same price as rubber from different sources and pick up your hard parts (balljoints, rode ends) locally from a napa dealer. However, for simple ease of ordering, a few extra bucks from a single vendor can make it easy to get an entire kit.
I tend to agree with CP about the need. I've heard enough horror stories on the web about shops you don't know classics in general and mopars in particular that it might be worth it to get a second opinion before dumping a few hundered bucks into a rebuild that may not be necessary.
The whole rubber vs poly is kind of a personal preference. Rubber certainly lasted a long time when these cars were driven hard and regularly so it would give you a very good service life now that your car is semi retired and not driven every day in all sorts of weather. However it does have some give in it and will provide a comfortable ride at the expense of some suspension responsiveness. Poly on the other hand can provide razor sharp suspension motion with the potential for squeaking and a noticeably firmer ride. Your ability to distinguish the two is dependant upon your driving experiences and seat time. To some it is obvious. To others they never notice other than to say that poly is squeaky and now their car creaks more.
