Will it be safe to take a shower using the medium setting until I replace it with a new heater tank?
Maybe. Perhaps the RCD will trip again when there is another fault. Or not. It's fairly likely to be safe. Or is it?
Personally I would not be wanting to use any appliance knowing that part of it had a fault.
With regards to the limescale destroying electric showers - unless you are going to have a water softener installed, there are two options:
a. Buy the cheapest shower you can find (well under £100) and expect to replace the whole thing when it breaks.
b. Buy a ridiculously expensive one such as the Mira Advance for £350, and then you can replace various parts when they break. Parts will be less than the whole thing, but could still be as much as the cheap shower.