I think there was a typo somewhere last night. for 12x7x4 size, it's about 2500 gals so you are correct. the filter amount should be measured with water in the bio chambers. don't count the settling tank. settling tank is just a mechanical to remove fish waste from the pond. the actual chemicals break down of ammonia/nitrates are in the bio chambers.
I would say pump with rating of 3000 GPH to 5000 GPH would be good enough. I would suggest get a 5000 GPH pump though. if you can afford more, then get bigger pump. imo, the bigger the pump, the better. keep in mind, you will not get any close to the rating number they provided so give yourself some overhead.
good idea to have a dock. you get a better view of the koi from top anyhow.
maybe you can use six 55 barrels as follow:
#1 and 2 - use as settling tanks. inlets from pond, outlets to prefilter
#3 - pre-filter. inlet from settling tanks. outlet to pump. you might need two prefilters if your pump is 5000 gph. if the water come in two fast, solids won't settle to the bottom of the settling tank.
#4 - bio chamber #1. inlet from pump, outlet to pond
#5 - bio chamber #2. inlet from pump, outlet to pond
#6 - bio chamber #3. inlet from pump, outlet to pond
can you fill the three bio chambers with whatever media you like. btw, you can bury the barrels to keep them out of eye sight.
have fun buidling your new pond,
Steve