Batteries should have the same approximate size and state of charge/discharge ability. If they don't, the weakest battery will damage the rest.
If one goes bad, how far behind are the others? Batteries only have so many cycles in them...
Tough call. If they were all the same age, like over two-three years, I might think hard about replacing all. Otherwise I may consider one decent used unit and cross my fingers.
No doubt opinions will be all over the landscape...