As a starting point I’d recommend taking a look at the Census data for the area which will will give you a general idea of the population in the region, from which you can make some assumptions.
In terms of your budget, we don’t have a specific budget set for the project, instead it will be up decide what you think is reasonable for the value it delivers, and depending on who the stakeholders are who would need to fund the tank project (including who pays for any ongoing maintenance if required, etc.). You can justify a larger budget, for example, if you can demonstrate value for money such as high initial upfront costs but the solution lasts a long time without maintenance.
I recommend reading our FAQs document, which can be found in the Resources tab – this document gives a bit more detail about budgets and how to progress through your design using assumptions.
Please reach out with any further questions!