I do not believe there is any intrinsic disadvantage in having differing instructors provided they are competent and conscientious. Rather, one could turn it into a positive experience by comparing and contrasting differences between them, understanding the commonalities underlying those differences, and using the exposure to a wider view of experience to which the multiplicity of instructors gives you access.
As far as elderly aircraft go, I recently took Mrs DRJAD and two friends on a round trip of Scotland and the Orkney Islands in a PA24 first registered in 1959. It had the best and most comprehensive avionics I have yet had the pleasure of using, was entirely reliable, performed excellently at the levels defined in the POH, and was a clean and comfortable aircraft. All in all, a pleasure to fly. A significant amount of the tour was, of necessity, conducted in IMC, and the aircraft performed faultlessly as a stable instrument platform.
It is not the age of the aircraft that you should fixate upon, but, as others have intimated above, the quality of the maintenance of the aircraft and its equipment.