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Solar pumps are an excellent solution for off-grid pumping. As always, you need good quality kit (Grundfos are very tough but we've also used Mono with good results), a proper design - matching head and flow to pump and PV array sizing - and an installation crew who know what they are doing. Once in, they run for years with no need of fuel and minimal maintentance. In general, they should last much longer than an engine based system because there is less to go wrong and far less need for operational input. Unless someone nicks the solar panels, which can be a significant risk if the pump is remote and locals don't have a strong sense of ownership of the system. A combination of local involvement / 'buy-in' and security measures - fixings and fencing should minimise the risk.
£40k could be a reasonable cost for the whole job, but it depends a lot on the head and flow required - do they want 500 litres/day or 50,000 ? I have no idea about drilling costs in Mali, and these again will be dependent on the borehole size and the geology, which in turn dictate the type of drilling rig and lining, and the time required.
You need at least to specify total static head, pipeline distance and daily water requirement to get a reasonable estimate of the system cost (excluding the borehole).