What you want from a tankless water heater, just like most other things, is for it to do the job - you need it to be up to the task of providing hot water for your household whenever you need it. To know what to look for you need to know a little about how tankless heaters work.
Because they do not use a tank, these systems have to heat the hot water as it comes in from the outside supply. In sa, gas tankless hot water heaters the mains water is sent through web of smaller pipes in the heater where heat is applied (a flame or some electical system). This means there is a limit to how much water can be heated at any time - if you send the water through too fast the water temperature will not be raised enough. In other words heaters will have a maximum number of galons per minute that they can heat.
You can estimate your hot water flow needs, and therefore know the tankless hot water heaters sizing you need, by adding up the need of all the units you will want to use simultaneously.
Faucets can use between three quarters of a gallon and two and half gallons a minute, a dish or clothes washer between one and two gallons a minute and shower units between 2 and 3.5 gallons a minute. This will be the volume required from your on demand water heaters
The other thing you need to know is what rise in temperature you will need to achieve. The average incoming water temperature is 50F, so you will be looking at tankless hot water heaters capable of raising the temperature by 70 degrees.