1. TNB is always AC. If you have a pump then DC pump slightly quieter that's all.

2. From the water hose if you can stop the water with your thumb easily that means low pressure. If you are already putting the heat max coz water too strong (i.e. cannot heat up in time) you should have good pressure already. If the min setting feels too hot then you should get a pump.

3. 4 unit Pana. But best to get older model DH3MS1. The newer model cut cost until Snoy's new unit also leak.

4. Normal usage 3.6kw model enough. There are 4.8kw models by Hitachi and Pana available if you are in cold places like Genting.

5. If buying online make sure the partner/online store is official dealer.
Should replace to be safe, it has been >20 years since its a National unit. Coz over time corrosion can eat into the heater element. Remember to use 10mA RCD as per ST guidelines.

For water heater it doesn't matter whether your electricity is AC or DC because it is a resistive load (to generate heat).

WH should last 10-15 years easily. At RM278/15 yrs (non pump model) its only 18.53 per year. Can throw it then as it has served its useful life