Information needed on pipeline replacement

We are a residential complex in Bangalore, India

We have 2 overhead tanks in 2 adjacent buildings to which water is pumped through a common underground reservoir. Presently we have 2 separate pipelines for both the tanks running in parallel till 75% distance and then the 2nd pipeline extends onward.s
The pumps have been in use for 15 Years and so I presume they would not be very energy efficient. 
We are replacing the complete pipe line as it has started leaking in several places. I was wondering if it is advisable to have a single pump and pipeline of adequate capacity for both the tanks to save capital costs as well as running costs.

At present we are using

  • 2 Nos. 10HP submersible pumps
  • Pipe for pump to overhead tank is 50mm dia & 300 m & 400 m length respectively
  • Height of the tanks from the ground is 6 floors so about 25 m
  • Presently we are using Steel GI Pipes
  • The temperature of the water will vary from 7C to 37C maximum

So my queries are your recommendations for :

  1. We are thinking of switching to Astral CPVC Schedule 40 or 80 pipes of 50mm dia. Can we use the same safely ? Will it last for 25 years as claimed by the company ?(The prices for the GI Pipes are nearly double then the PVC Pipes). So should we use Cpvc Pipes instead of GI Pipes
  2. To go for a single Pump and pipeline

Thanks for advising