I doubt you would have to change the gas meter/line if you went with a gas furnace and gas WH. The nameplate on your WH lists an input of 150,000. So I assume the line is sized for a minimum of that, maybe a little more. If you look at your AHU rating, I bet it is less than 80,000 BTUs/hour. A high efficiency furnace would give you 80K out with only 85K in. That would leave you with 65K for a WH.
I still would like to see a resolution to your current challenge. I just wanted to point out that it looks like you have an adequate gas supply should you have choices to make later.
I usually heat only part of the house (900 sq ft?). In the daytime, I use the unvented gas logs. I have them throttled down for a small blue flame (no yellow flame, no unburned carbon, less chance of carbon monoxide) for a small slow steady heat. I went out and timed the meter and found I was using 17K BTUs/hour. This amount will keep the 900 feet at 70F with outdoors at 20F. So 50K would probably heat your 2400 feet.