Hello Andrew,
This sounds like a splendid idea. When I built my first pond, I dug a hole, lined it, built up a waterfall with a lining behind it and threw in a pump and was on my way. Needless to say I learned a lot--the hard way. I think that an interactive program where you could draw the area where your pond would be including elevation/slope would be the starting point. Then have the choices for construction--concrete, flexible liners of all kinds, rigid preformed. Be able to click and drag those into the picture. Of course, you'd need filtration systems and their placement, pumps and their placements, waterfalls and other features (fountains, etc.), plant choices, bog placement, choices of bog plants, stream channels and other feaures such as springs. YOu'd also want to have a way to show the adjacent buildings and plantings of trees and the shade they will cast. Having all of these things organized into categories which one could click and drag into the picture, along with suppliers and power requirements would be a wonderful tool for backyard watergardeners. Oh, yes, a volume calculator, a formula for figuring the cost of operation of the pump and lighting. I love the idea, but it sounds like a massive project, researching all those sources of equipment,
Once you have the features placed, you could have warnings that would indicate that certain features would either not be safe or would be more costly than necessary. There is a plethora of programs that deal with landscaping and home remodeling, but I've never seen one that deals specificially with water features. You might get some ideas from some of those home improvement programs.