For bathtub size, you need a tank heater or solar heater.

i think maybe enough.

you dont need to fill the tub all the way to the rim. maybe half the tub also enough cause once the body goes in, it displaces the water
and it also depends how hot do you want it to be. less hot water mixed with more cold water = in the end not very hot.

but i cant really say cause no recent experience. my old house i put in 50L but I never once try the long bath so I also not sure whether enough or not. 50L capacity was recommended by the shop
In the end I think wasted to put in too big cause you will burn more electricity 

Instant heater cannot use for long bath - not hot enough, takes ages to fill the tub, by the time it fills the tub, the heater also catch fire already 

that link you gave doesnt say how many litre of water that tub can hold
anyway for joven storage heaters you can see at the same shop = http://www.ewarehouse.atkc.com.my/index.ph...joven%20storage
the prices between each capacity is not a big difference

this storage heater is normally install above your bathroom ceiling board. each bathroom one heater. for bathroom with only shower, a 25L tank is enough
you need to find out if your bathroom already have copper piping for hot water and also electrical point for the tank. Also find out how much ceiling space you have for the tank to fit up there

you said you are buying a new tub, are you replacing or adding it in?

I only turn on my heater just before I go in and shower and off it once I am done or after the next person is done. It always have left over hot water inside the tank so you dont really need to like wait 20 mins for water to heat up. Only the first time you use it, you need to wait.

For normal household people, there wouldn't know about this. And don't think the manufacturing will tell you when need maintenance.

Till the unit spoil.