John,
ok, what's harry potter?
I am not an expert but let me try
your 12x7x5 will give you a 4450 gals size pond. filter should be around 10 - 15% of pond size so that works out to be something in between 445 to 667 gals. of course, this it really depends on your fish load and weather you use bead filters or bio chamber filled with media such as matala/japanese mats or combo of bead and media or some other media in poly barrels.
based on your drawing, if those 3 filters are 55 gals barrels, it's might not be sufficient as your fish grow say 3 yrs from now and beyond. also, I would think the pump will likely to starve of water if its intake only come from 3rd barrel. check out my new pond design to see if it's something you want to consider
http://koi-bito.com/forum/showthread.php?t=4669
for pumps, I recommend to purchase a pump that at least equal to your pond size, preferrally double your pond size. for example, your pond is 4465 gals, you should get a pump that deliver at least 5000 gals per hour. you may want to consider two pumps for redundancy with 3000 gals or more per hour. if you are looking for whisper quiet pumps, then purchase artisian performance or sequence primer pumps. my friends are using them and they are whisper quiet. if you don't care about the noise, then aqua flo pumps will do the job. if you want purchase online, I can recommend a very good place for you. you might even get some discount. PM me for details if you like. I am not a seller.
your bottom drain layout looks fine for 12 ft long pond. you can use two 3" and connect them into one 4" pipe before feed them into the vortex settling tank. on top of the vortex settling tank, you can have a 6" matala to trap little sediments that did fall to the bottom of the vortex. this helps the pump and filter from clog up with unwanted wastes.
depends on where you live but the 1" plant shelf might provides a platform for heron to stalk your fish. if you want to use plants to help filtering the pond (btw, I think using plants for helping pond filtering is great), you should have a bog garden and have the water overflow from the bog garden into the pond. this way, no heron can stand up there and stalk your fish. besides, koi are know for biting on plants. btw, imo water hyacinths are best but I was told that peppermint plants are the best. have not use peppermint plant in my pond those. water hyacinths in the same pond with koi is asking for a lot of clean up. koi bites on its root and create a mess so have a separate bog garden for this.
I don't know what you meant by "dock" in your drawing. is that for the waterfall? personally, I like small water fall because I like the sound of the water fall run. I don't like the noisy waterfall. big waterfall also create a choppy water surface in the pond. i enjoy the tranquility more I guess.
what type of media do you plan to use for bio filters?
Steve