Just finished making a 1000 gallon koi pond. I love my koi and worked my butt off making a nice big house for them and ultimatley want to make a 6000gal pond. Got just about everything used off a guy who was moving and quitting the hobby. We were EXTREMELY lucky. The equipment is the expensive part and if you want your pond clear and fish alive you have to get all the parts to make it work. I started with the equipment so I knew how much to dig and how much pond liner to buy etc.
When starting this process unless your hiring a professional TAKE YOUR TIME. Don't rush any process. We did all plumbing, electrical and hardscaping a 3 ft tall above ground brick perimeter and 4ft tall waterfall inside.
Look at pictures of peoples ponds find out what design you want and will be happy with such as waterfall ? places to put plants? ground level? above ground level? The 1500 gallon koi pond on the link below will walk you through pretty much everything you need to know.
My pond has 2 water ins (1 jet that can have an aerator to hook up to it for more oxygen in the hot summer and 1 waterfall, not just for looks but circulates and aerates water). 2 water outs bottom drain to suck the sunken debris and makes it easy to empty if needed and a skimmer that is aparently a pool skimmer but works). all ins and outs are hooked to jandy valves which I recommend because it allows you to adjust the water flow on ins and outs as you want them and can turn off waterfall/jet/skimmer/bottom drain individually if you want.
This is the best resource I have found describing elements for a koi pond.