If you mean up to 80 degrees C, the laws of physics are against you.
A 'decent flow rate' would be say 15 litres per minute?
Say your incoming cold water supply is at 10 degrees C, and you wanted hot water at 80 degrees C, that means heating water flowing at 15lpm by 70 Degrees. That takes about 65 kW according to my reckoning.
That needs a big 3 phase electricity supply, or a big gas pipe!
If you could manage with a more modest flow of say 10lpm, heated instantaneously, that would still need 44kW.
You would stand a better chance of getting both high temperature and high flow rate with a stored hot water option.
As per Undertrained, giving more info about what you want to do would help.
EDIT: Were you thinking of going electric or gas for this?
Maybe you already have a 3-phase electricity supply, or a larger capacity gas meter/supply?