Koi will be fine in 24" in USDA Zone 4. I would recomend at least 30" depth with 36-40" even better. The filter does not need to be run year round, but some sort of areation near the surface needs to be employed. After the water dips to 40-45 degrees, feeding stops altogether so their will be less solid waste. Biggest concern is the gas exchange, and that is easily addressed. A pond of 3000 gals. is a good number to shoot for. There is no way you will only keep 2 fish. The pond will look sparce and my Koi seems to really like swimmming with his goldfish friends. If your friend breeds them, pick his brain for ideas. I'm assuming he is local?