Shelley,
You can always check with the local county extension agent, but basically in our area I believe it's 14 to 16 inches deep. Another thing to do is before you put the sand and liner into the hole, dust the sides with a good coating of lime to keep underground varmits from burrowing under and into your liner. That tip comes from an established expert on ponds who spoke at our last Tropical Fish club meeting.
So your pond should be a minimum of 2 1/2 to 3 feet deep. Given the fact that you live in a rural area it is suggested that your pond's sides should go straight down rather than sloped or with ledges. That will keep most of the four legged varmints from using your fish as a snack. To support potted plants place one of those square blocks used for making walkways with under the liner. Place the liner over it and then place the potted plant on it. Make sure that you cover the top part of the pot with gravel to keep the fish from digging into the dirt or mud the plant is in.
You can also take one of those quik set kiddie pools to place in the pond to act as an additional barrier to keep any plant roots from puncturing the liner of the pond.
/ \\
/------------------------------------------------------ \\ (Frost Line)
/ \\
/ / \\ \\
/ / (Plants) Kiddie Pool(Plants) \\ \\
/___ /__|____|__________|_____|____\\____\\ Bottom of pond liner
|_____| |_____| Blocks for plant supports
Leave room around the sides of the kiddie pool so the fish can swim around it and over the sides of it. Or you can use the space around it to place potted plants and leave the center open for your fountain. Hope this helps give you some more ideas.