You are thinking about a very similar tank to one I already have. Here are a few thoughts.
For substrate, go FLOURITE all the way! I have heard a lot of discussion about Eco-Complete and some of the problems with it and I think Flourite is the better choice. The only problem with Flourite is when you initially set up the tank it likes to make your water muddy red for the first couple of days. My tanks are almost all Flourite, but I do use FloraBase in a couple of the smaller ones. It works very well for small tanks, but for a 20, I would definately go Flourite.
For lighting, I have one 65 watt CF on the back and a standard 10 gallon strip light on the front. It seems to be a perfect setup for the tank.
For CO2, go with the Hagen ladder and 2-liter DIY yeast culture. That's what I have on my 20 gallon tank. I don't know what people are talking about when they say it is a hassle. Once a month I drop in some sugar, water, yeast, and baking soda and it cranks out the CO2. It is very low maintenance and very low cost.
As for the open top tank, I think they are cool, but I will never have one because of fish loss. By the way, I give you one month before your final hatchetfish jumps out of the tank. You will lose them all in an open top tank, so I would consider something that doesn't sit at the surface and jump long distances in the wild.
The last thing I would suggest is possibly using 50% RO water with your water changes. You can pick it up at any Cub Foods store, if you have them in your neck of the woods for like $.30 per gallon. Your CO2 will bring the PH down a little, but I have had no problems with medium soft water with a PH max of 7.3, whereas I had lots of problems with the hard water out of the faucet with a PH of 7.5 to sometimes close to 8. That is the least of your concerns at this point, but I just thought I would mention it.
Two 1 GA desktops, a 5 gallon, a 2.5 gallon, a 10 gallon, a 20 gallon, a 25 gallon, and two 75 gallon tanks, all planted.
I am an expert on algae, so ask me if you have questions. I know how to grow it, just not how to get rid of it.