A key specification, not listed in the Rheem datasheet, is pressure drop at GPM of flow across the heat exchanger. Of course it is important for any plumbing to not restrict flow, but this pressure drop can be significant and is a key figure of merit along with temperature rise. Noritz NRC1111, which is the only heater I have any experience with, has low pressure drop, high flow and high temp rise.
You will need the largest heater available if your water is in the 45 degree range. No worries, they only use a lot of gas when you use the water. But you will need to feed it with a large gas pipe and gas meter. If this equipment is marginal in your house, you'll have to upgrade.
I don't have personal experience with reliability on that brand. If a water softener is an option, it will help keep the heat exchanger free from scale.
Edit: No experience on noise with that model either. Most tankless are reasonably quiet. Mine is in the attic above the master bath, so not that far away. I can barely hear it, even in the bathroom directly underneath. The water running in the shower is much louder. But use common sense and don't mount it directly over the bed. Also, noise can carry out the exhaust vent.