Hi sifus,

I have a built-in rain shower head from the wall and I woud like to retain the rain shower feature.

To do that, do I still need to buy the rain shower model water heater or non-RS model will do?

If the water heater installer guy or you can DIY yourself according to above.

But if you plan to have the hand shower then you have to place the order with rain shower which is external type ones.

Common instant water heaters (3.6kW to 3.8kW) usually don't generate enough warm water for a satisfying rain shower usage. Plus instant water heaters' warranty will usually be voided if you use shower accessories that do not come with the instant water heaters, like what you are trying to do.

If you want to use instant water heaters, either get those multipoint type or any that is above 5kW. I think one of the highest powered instant water heaters that is SIRIM-approved is CENTON Serene Highlands with 5.4kW. U can just search for it via Shopee.

There are always limited temperature rise an instant water heater can achieve. If you like to shower with super hot water like those hotel kind, better get storage water heaters that outputs above 70°C hot water, using a mixer (the lever on your wall) to mix hot and cold water together. But prepare to spend thousand plus for storage water heater and its installation charges.

Usage of 3-pin plug+socket connection in the bathroom is against Suruhanjaya Tenaga's guideline, endangering the users. We suggest u to use:
1. a direct cable connection (bypass the 3-pin socket), or
2. replace the 3-pin socket with a Splash Proof Connector (like the one on @pawpaws 's wall). Good Splash Proof Connector brands are CENTON or STEC that can be easily purchased from Shopee.

RS model WH.
But bear in mind it will not be as warm as regular WH wirhout RS kit.
Your cable in wall 2.5mm² typically only good for 3kW class heater.
If you want to install 4kW or 5kW you need 4mm or 6mm² cabling from DB box respectively because the current draw very big, it will overheat undersized wires.