alt Hacker NewsHEPA is less important in most scenarios than "something that on a few passes will catch everything", which MERV-13 will do. Not all systems can handle the pressure drop of a MERV-13 but many can, and there are some pretty low-drop options out there if you search for them.
13 is better obv, but merv-8 is a great drop in for most systems, and it makes a huge difference. my system runs the fan for an hour every evening, and that also has been an improvement.
i have to do a major upgrade next month (30 year old gas furnace!) and i will need to do some research, but i would like to even ascend to true HEPA if it's feasible