When you say rainshower are you referring to the shower head (rainshower style) or the placement of the showerhead (shower head that descends from the ceiling)?

We have both a rainshower type shower head that extends from the shower arm on the wall as well as one that descends from the ceiling + a handshower. We rarely use the one that descends from the ceiling. We have a diverter valve that allows us to have just 1 of the three on, 2 of the three on or all 3 on at the same time. The water pressure does go down slightly when you turn on the 2nd and 3rd heads, but all are still easily usable. We really like our set up and wouldn't change it. Ours was the floor display as the finish was being discontinued so we got a great deal, but I think it ran ~$700 retail.

You will need valves, control trim kits, shower heads, diverter, handshower, elbow for handshower & either a clip or slide bar for the handshower (we went with the slide bar for greater flexibility)--I'm sure I'm missing something. We went with Delta. As far as plastic, the handshowers are usually plastic with the same finish as the control trim. If it was metal, you risk breaking tile/chipping the shower floor if it falls. Much cheaper & easier to replace the shower head. Pressure in the rainhead will largely depend on the water pressure in your house and how many other water sources are running at the same time.

Hope this helps!