Good for your wife, I dug my own pond, too.
That size is good, and definitely go a minimum of 36". I've heard that winters in Mn. are particularly cold, and deeper is better, I just found a link that discusses Minn. ponds.
You'd be better off if you went to 4', in fact that's the minimum requirement for koi. They need deep water for swimming year-round. And koi need a mimimum of 1000 gallons. Calculating that size pond however, at 36" deep, it would not be large or deep enough for koi. There are numerous sites that help you calculate pond size and volume, like this one:
If she wants potted plants, she should dig shelves along the size of the pond, somewhere around a foot deep, that's what I did. The pots rest on the shelves. Of course that will lose some more water volume. And koi will knock the plants right off the shelves if they aren't weighted enough, koi are rough on plants. There is nothing wrong with goldfish though, in addition to comets there are sarassas and shubunkins which are colorful, with long, flowing tails. So if building a several thousand gallon-sized pond is a little daunting for her, goldfish would do great in a smaller one.
I'm not sure about the bog and filter aspects, I just have an underwater pump and filter in my pond. I've see numerous DIY threads on pond pumps etc. on Koko's goldfish forum, including pics, it is not difficult to do.
While googling Minns. ponds I came across the link below which describes pretty much all I was going to say.