Can use class 7 pipe + inline ball valve to regulate the pressure so it will be good pressure and not excessively high.
RM1800 seems on the steep side. I would say the risk of having the water running out on the main tank is rather low but good pump will have overheat protection. Your situation where by the water tank is higher than the unit is easy to pump so a small 150-200W pump will suffice as your water doesnt need to fight against gravity that much.
The entire surface area of the upstairs tank is easily 1-2 sq meters (if small tangki) is enough to push down the water into the 1" pipe.

A "150W HITACHI WM-P150 XS AUTOMATIC CONSTANT PRESSURE WATER PUMP" should run you around 660 for the pump only. Key is automatic pump so no need to turn on turn off.
You can backflow it from another outlet using water hose (tap to tap)?
Old high rise >15 years yes, iron pipes corroded and from 1" become 0.2" diameter left for water to pass thru. It could also be the WH filter clogged/flexi pipes clogged. Usually management will replace with new piping.

New high rise within past couple years no.. water pressure is so good I need to max out the WH to have enough heat to take bath.

Depends on the pump type. Cheap pump will get water jetting when pump automatically switch on and off. it is because the pressure switch turn motor on and off abruptly. When motor is turned on the pressure build up in pipe but then the water out is less casing pump to switch off and when pressure low again the pump on and so on, causing water jetting. More expensive one have bigger bladder tank thus less water jetting effect.

Nowadays, high end pump is using inverter to vary motor speed, thus the pressure in pipe is consistant. This will let water flow smoothly. The inverter type even no need to have bladder tank.

The RM1800 probably this high end type. You need to ask. I know that Grundfos Scala2 is around RM1700~1800. This is real constant pressure pump.

Grey PVC pipe the higher the class the tougher the pipe due to thicker material. Class 0 is low class and should only be used for light duty (aka aircon drain pipe). If wanna bury in concrete use class 7 direct, its not that expensive but will save a lot of headache later on.