Hi there, I would choose a company, who has a long history in manufacturing tankless water heaters. Then I would make sure, that the company installing the system is very familiar with this type of water heating system and supply their own written warranty on their installation. You didn’t mention, whether your home comes already with a gas connection, as a later connection to the house can be costly. Typical information you will want to know, before making a decision on tankless water heaters: does your home has an existing gas connection? number of full and half bath rooms, size of family; do family members prefer showers or bathes? How many showers/bathes simultaneously? Is your washing machine energy efficient (front or top loading); How many electrical water-heating devices in your home have a built-in water heater? Details of the companies warranties on parts and labor, size and design of the water heater.
Here are some websites with lots of good information on everything you need to know about this topic.
Stiebel Eltron & Takagi: http://www.tankless-water-heater.com
Quote: “Tankless Water Heaters by Stiebel Eltron, established in Germany in 1924, and Takagi, established in Japan in 1946, are the most reliable and well engineered electric and gas tankless water heaters made anywhere at any price. Remember that Germany and Japan make the finest automobiles and electronic products in the world and it's no surprise that they lead the world in water heating technology as well.”
Bosch:www.cpotanklesswaterheaters.com They also make beautiful front-loading washing machines and dryers and in comparison offer more for their money than other manufacturers.
Tankless water heaters by Seisco: www.seisco.com I also would consider to install a separate small under-the-sink unit in the kitchen and bathroom sinks. One more thing: Some companies also suggest keeping your existing water heater as a back up. I hope this was helpful. I would appreciate your feed back on your experience.