Sounds like a great opportunity to learn, and expand. Going deeper will save plants from freezing, and if you do decide to add fish, you’ll want at least 3ft deep. Keep in mind, the rule of thumb for pond fish is 60 gallons per goldfish, or 1000 per koi. The 1 inch per gallon rule only applies for nano fish like neons, bettas, guppies, very small fish with out a lot of mass.