Hello. When I first started reading this, i thought to myself, she is making a hug mistake! If I understand correctly, you haven't started your NEW pond yet have you? If not, I would suggest investing another $70-80 into getting a bottom drain for that pond. When I first got into koi ponds, about 6-7 years ago, I didn't have a bottom drain in it, and one fall I got the flu, and couldn't clean it, and that spring was a PITA!!!! It had at least 6" of poop, leaves, and all other nasty stuff! I still don't have a PROPER BD in this pond, but a DIY, and it works OK. The reason i don't have a real BD yet is because in order to do that, i would have to move 3, 1.6 ton boulders, and I don't want to do that at the moment.
You can get a 3" PVC bottom drain here,
http://waterartsconsulting.com/bottom_drains__intake_screens, just over a little halfway down the page.
Also, get an external pump. They are a little more expensive to buy, but they use 1/2 the energy of a submersible and they get much better flow rates.
I would not suggest using the waterfall as your only filter. If so, buy a couple of bio balls at HD or lowes, and put them in there as extra bio, but DO NOT clean them off when you clean the filter. rinse them in a 5 gallon bucket of pond water by swishing them around, but don't let them touch chlorinated water.
I don't know how many gallons you are planning, but if you want koi, I suggest a min. of 1,500 gallons,(2,000 would be a lot better) and 4' deep. To determine how many gallons your pond is, L x W x H x7.5.
Koi grow to their environment, and they can grow up to 4' long. The general rule of thumb is 10 gallons for every 1" of koi, or 300 gallons per koi.
Also, if you don't want green water, get a UV light. They do wonders on getting rid of all the algae, AND if you get one rated high enough, it will kill any waterborne parasites.
I hope this helps you a little bit, and if you have any questions, shoot me a PM or email,
Eric@KentuckyKoi.com, or post here, and I will try to watch this thread.
Thanks,
-Eric