Got a quotation from a Grundfos recommended plumber to install CM3-5pc at RM550. Just wonder whether this price fair or not?

Normally these water pumps are rated to IP55, which shall able to withstand even water jet from all angles technically! Rain water shouldn't cause too much harm to the pumps!

Some submersible pumps can be installed at dry pit without any problem.

PC or PT? Couldn't tell without it! If PT, I guess you need to ask technician to pump the air every one or two years. The water pump itself just run until it dies, then change new one, repair cost might be similar to changing the new one!

If I am not mistaken, PT comes with a hydro-pneumatic tank look like attached photo, PC no tank. Your photo look like PC series.

I do not own any Gemini Pump, but normally household pumps do not required maintenance until the pump run to die after warranty period, except those pumps with hydro-pneumatic tank which need to check and pump back the air pressure in the mentioned tanks every one to two years.
If you look into the details, components of pumps that may give problem are bearings (nowadays mostly maintenance free), mechanical seals, pressure switch and pump controller. These parts only need to be replaced when they failed, but the repair cost (parts + labour) may not too cost effective than getting a new pumps.

If I am not mistaken, SMART Tunnel is using Flygt pumps in most areas.

Alternatively can get noise silencer manufacturer to custom make a noise enclosure for you, such as Istiq, KSM, Belflor and Trox.

JF Home Appliances USJ