Look at Bosch. I hear those damn Germans are good at mechanical stuff...
There are charts that tell you exactly what size you'll need. You need to know your average incoming water temp, gph of your municipal system and anticipated usage (family size, bathroom/sink/laundry count, etc). You most likely will have to upgrade your electrical circuits/wiring to handle the load (assuming we're talking about an electric unit), as some of these units use
double 80a breakers
Don't let the (inevitable) naysayers discourage you. Tankless is the way to go.