My pond digging experience wasn't too bad. I did give up after my shovel was bouncing back at me every time I hit the ground because the ground was so hard, but no too many rocks. Lots of clay though. I filled it in last year and have decided to go with 3 level above ground pond this year.
In this climate I would not worry about a bottom drain. It's no worth it as we usually do not go deep enough to get under the frostline. A good pond vac is great.
Algae is not much of a problem if you keep enough plants in the pond and even better if you have a marsh area to plant even more plants. The trick is to keep the soil out of the pond as this adds lots of nutrients. I used a mixture of barley straw in my filter and concentrated barley and never had any problems with algae. I also used a UV to get rid of the green water when it got really bad.
I miss my pond and hopefully my multilevel design will help keep my plants apart from the fish and apart from the turtles. One thing kept eating the others. They don't do well together I find.
