Welcome to the forum. All great questions and you definitely will have more. Best to do the homework now like you are doing rather than in the middle of the build. Be prepared for changes to your plans, as things do happen that you were not prepared for.
You may want to dial back the number of koi you are planning on. I will defer to
@Lisak1 on a safe number as she is really good with setting people's expectations when it comes to the number of fish you should have. Your pond is probably going to be under 4k gallons, at this stage it is hard to calculate exactly how many gallons you will have. It will depend on your shelves and the stones you use that will cut into the volume. I thought I was going to be over 4k gallons in my pond which is 25' by average of 12' wide and 3.5' deep, some of my shelves ended up being wider in some spots, which looks great to have variety but it cuts down on the volume. I used a water meter when I was filling it and ended up over 3700 gallons plus about 400 in the bog. I just filled it in September so I did not fully stock right away, just added 5 shubbies to start with. I will have some koi but will limit them (because I fear the wrath of
@Lisak1) Just kidding Lisa
Do not skimp on the liner or the pump. Everyone always say to try to go larger on the size of your pond then you initially think because when you are done you always wish it was larger.