You're wise to stay away from the tankless for the reasons you have given and more. The GE/Rheem is a good choice in water heaters, and I'd stay with the vent system you currently have. I have a power vent and it is great, but I removed the chimney that had been used for venting an older heater and now I am committed to a power vent or direct vent forever...which is fine except they are quite expensive. If you are doing a basic remove and replace and don't have to move the gas line, there isn't that much to installing a water heater except for handle the beasts. Some plumber hard plumb the water lines, but those I know around here use 3/4" flexible copper which makes connections a breeze, just don't kink them. As far as size is concerned, since you are anticipating a family in the near future, a 50 gallon might be a good idea. A 40 gallon might be OK, but with that additional 10 gallons, you would have a reserve supply.