Any particular reason you need a very small car? Smaller cars in Sri Lanka are often perceived to offer massive cost savings when compared to something slightly bigger, hence the prices are pretty mad. In reality though if you pick right, the economy difference isnt all that huge. So unless you have a specific reason that you need a very small car, you would be better off looking at the slightly larger options as well.
Be advised that since your budget is on the lowish side you wont be able to get very many newer cars, so if a new number is important to you im afraid you have a problem. The Minica that you mentioned is a VERY rare model in Sri Lanka and spare parts will be tough to hunt down. This is probably the reason it had that price, which is VERY low for a car with a 300- plate.