I think the 300mm PF + Nikon 1.4iii TC is a very good option if price is your major consideration. I also have the 500mm PF and I use it a lot when in the field photographing wildlife where I usually cannot get close to the subject here in AZ. However, when I can get close (bird sanctuaries, wildlife preserves, zoos, backyard) or for longer hikes in search of subjects the 300 PF with the 1.4 TC in my pocket just in case is a really good set up. Super on your D780 I would think.