Adair this is exciting!! I am sure it will be beautiful! As long as you're willing to put in the time, I don't think you can go wrong with one. So look forward to seeing it and its progress!
My own garden going through some changes as we speak. The main central area is being rethought. The round stone wall is gone where my Mirai elm sits. It will now be redesigned with large stones and a few heavy posts where I will be placing my largest trees. Pics to come at some point. I hope it turns out well. As I am clearing up the entire area, I am once tempted to put in a koi pond myself. My main issue is that I do have all sorts of creatures roaming around including bears and foxes on the ground and hawks and blue herons up in the air. I just think the koi fish will be sitting lunch for my wild friends.
Another issue is that it will need to be quite deep to ensure they survive the winter. Let's see what else...