I just built one but I am by no means an expert...
i can only help out by sharing what i have learned as i built mine all on my own and saved ALOT of money that way.
To dig one yourself recommendations on size for a koi pond is 1000 gallons (my 2 pond-upper and lower levels-setup is 700 gallons total, close enough for me) and to have areas at least 3 feet deep to allow the fish to avoid predators and to find cooler water in the summer.
on mine one side slopes/gets shallower because i have frogs but the other half has steep edges which is recommended to once again protect the fish from predators. there should also be overhangs (like big flat rocks) to allow for shade and it also thwarts predators like raccoons, i built a little alcove in the deep end of the pond that i then covered in rocks to make a cave, the fish love the cave.
a fake heron by the pond has been known to scare off real ones as they are territorial.
A fence around the pond will help protect the pond from cats and others like raccons-who may still climb it but at least the fence may make them change their mind or at least slow them down. not to mention keeps kids or pets from drowning.
if kingfishers and herons/egrets are an issue a net hung over the pond about a foot or 2 over the water will keep them at bay, kingfishers cannot dive through them and herons/egrets do not like to go under them and its hard to sit on them and fish.
a pond pump or filter is needed and should circulate at least the same amount of gallons an hour as what is in the pond (1000 gallon pond needs a 1000 gallon and hour pump)
koi like to dig, they are bottom feeders and so any plants need to have rocks in the pots to prevent the koi from up rooting them.
that is what i have learned, hope it helps you out w/planning your pond.
