Before you decide on a GPU, have you looked into the 2.8 Evee render engine or third-party renderers? How are you rendering these complex scenes? Have you divided them into layers at all? Depending on how much rendering you are doing you may want to look into something like sheep-it render farm where you can earn points when your machine would otherwise be idle and then farm it out once you know what you want for output. This usually makes more sense for animating rather than still shots though.
