i see your looking at a pond in the 2,800 - 3,000 gallon range...if your going to stick with 4 - 6 fish. you can easily buy them all at once. as long you dont get large ones. like i said earlier...you control the waste production by the amount and frequency of feeding. my first pond was only 450 gals. and i had 2 dozen fish in it within 3 weeks, in 6 weeks i had over 30. 18 of which were 5"& 6" koi. (not that i am recommending this). first week= 1 feeding a day ...2nd week= 2 feedings a day, 3rd week on = 3 feedings a day.....never had a bad reading. koi can handle a wide range in ph, just slightly less than goldfish...ph swings are whats dangerous to both, more so with koi. a little higher or lower ph thats stable is far more acceptable than a ph that fluccuates. you can start off the system with some beneficial bacteria to accellerate the cycling process. on the plant situation i would reccommend building a floating plant cage maybe 1/3 the size of the pond. water hyacinth and or lettuce thrive in them and really pull the nitrates from the water, they have flowing long fiberous roots, and reproduce at a good quick pace.