Diesel is pretty much available in most places. If you can see a lorry, the chances are it's running on diesel, hence diesel must be available within 1/2 a tankful of the closest place that you've seen a lorry. I'm not saying its the best or most sustainable fuel, just responding to the question on availability.
Security can certainly be an issue, diesel engines generally have fairly high resale value as they can be put to many uses. However they are quite heavy, hence difficult to steal. Diesel itself can be stolen much more easily by syphoning. The level of security required will depend on local crime rates and the sophistication of organised/petty criminals operating in the area.
Do remember when considering secure enclosures that your engine will also need adequate ventilation, both for air intake and exhaust. Our handyman built a secure cupboard for our emergency generator without doing any calculations... Its ventilation windows are incorrectly located/sized. As such, the generator will only run with the door open. Not a huge security issue as it is very loud and runs hot & so we know as soon as it stops & can get out to lock it up before significant risk of theft, but you can't beat a good design.
One of the biggest issues is the cost of diesel. In motorised schemes where revenue collection is weak there may be long periods of time when pumping does not take place because no fuel has been purchased by scheme operators. Before constructing such schemes it has to be clear whether the community can pay / are willing to pay or if anyone else is going to subsidise purchase of diesel.
Solar isn't necessarily without its problems either. Without some sort of umbrella organisation that provides ongoing training to operators, credit to operators for purchase of expensive replacement components and access for operators to high quality spare parts, any solar scheme will most likely operate below capacity in the long term and may fail all together. If pump capacities have been poorly matched to borehole yields, possible pumping hours and storage capacity then schemes can struggle to deliver water when it is needed most. Politicians can also make revenue collection difficult for operators by claiming that water should be free of charge.
Solar is a fantastic alternative to diesel only if efforts are made to establish a viable umbrella organisation that can support operators with the many challenges that emerge as a result of trying to manage a scheme. It is not enough to train operators and leave them to run schemes alone.
Some schemes make use of both solar and diesel. This involves additional costs and complications.
This thread is public, all members of KnowledgePoint can read this page.