Interesting read John,
We have two new members in our koi club, one on Mk 2 the other in Mk3 thank God they both met me before starting as they were still about to make huge mistakes! Im not tying to sound big headed! but ive wasted thousands doing it wrong and if I can give sound advise to help a fellow koi keeper I will, I think most of the time it comes down to saving money, personally I wouldn't even bother until you got the money in place, as frustrating as it is, it will be even more frustrating when you run into problems! The only thing I can add is to possibly build it indoors, or at least have an option to be able to make it a room in the winter, it extends your hobby to all year, ambient temp are better and less contamination from the sky above. As for shimmers, we have the softest water probably in the country, and we still get them 😉