I was gonna call Rinnai to verify this, but I think you need to have it installed by one of their certified installers or you don't get a warranty.
Might check that first for other brands as well.
I installed a gen1 Bosch years ago, worst part was running the double wall vent pipe through the roof. Also, you should check your gas flow with a pressure gauge to make sure you've got enough supply.
They aren't maintenance free, your'e supposed to flush them once a year just like a tank style. The Bosch sucked - not sure if it was because it was a gen1 (15 years ago) or because it was a Bosch. Had another house with a Rinnai and it worked great.