Tankless water heaters are a great choice for always having hot water. But like speaker73 mentioned, you'll have to upgrade the Gas line to 1"-1 1/4" depending on the distance from the Gas meter to the tankless. I suggest going inch& 1/4". You may want to consider placing it near the shower, so that you have instant hot water at the shower, but you can always just get a booster pump if that doesn't work.
I've installed a few my self, let me know if you have any questions, txt me if you still have my number