Welcome.
I too think it will be too shallow, unless it is sheltered from the sun. Check with your local council as to maximum depth before fencing becomes a requirement. It is usually around 45cm.
Lilies need at least 45cm I think (most of them). My pond is full of oxygen weed I pulled from the river that runs through the middle of my town - free
Visit your local garden centre and see what they have in the way of aquatic, marginal and bog plants.
How are you converting the sand pit? What will the pond be made of? Concrete or liner?
If it is a liner, and you have no chlorine added to the water supply, it will be a case of fill 'er up, wait for the temp to settle (only takes 24 hours) add the plants and fish. Goldfish will be starting to spawn any time now and mid to late Dec onwards is the time to find young ones really cheap.