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In general a high quality HDPE tank (roto moulded) should have a life span of 10-15 years. A ferrocement tank may last as long, but it also may well sprout some leaks and need repairs sooner. You could argue that this might therefore be more sustainable as repairs can be made locally, but my experience they rarely are.
At a quick check with a Nepalese colleague in the office he thinks plastic tanks are available locally (in the cities) and that these would come in from India. So you will likely pay a premium and may also have transport to arrange.
I have asked a colleague from our office in Kathmandu to add more.
Hope that helps