sounds like what you're looking for is a thermostatic shower valve working off a pump. A standard power shower would be cheaper. Take a look around the bathroom showrooms and get an idea of what is available in both lines and take a note of prices.
You're probably talking €300 plus for the pump and another €300 plus on the thermostatis shower if you go that route. This option will mean you must have hot water available for the shower. You could get a decent power shower for half that total.
It's all down to how much flow/power you're looking for, and whether you have a good supply of hot water.
Leo