Submerged stems need no substrate. For lilies, rushes and other marginal and mid-level plants, you just plant them in the pot. You would need very deep substrate to accommodate the root system of a hardy pond lily or lotus, and that would decrease the amount of water in the pond, necessitating a lower stocking level and hotter water in the summer.
I have no substrate in my pond. If you are using a pond liner, I'd have a hard time seeing the need for a substrate.
Is your water uncommonly acid? Why do you feel the need for a buffer? Ponds being larger than aquaria and outdoors are generally much easier to keep stable than indoor tanks, but not if the pond keeper is constantly fiddling with the pond.
The number of fish depends on the kind of goldfish you want to have. Plan on 50 gallons ( ~180 liters) per for comets, commons or shubunkins, half that for fancy goldfish.
If you have a temperate climate without many temperature swings, you could probably house fancies year round in your pond.
ETA: Make sure you have lots of floating plants in the pond. They help regulate temperature, combat algae and increase oxygen available to the fish. Common floaters include water lettuce, water hyacinth (where legal), azolla, salvinia minima, duckweed, frogbit, etc.