But how do I even receive the carriage return?! will it be a ascii-character like the others?!
Yes. The carriage return and line feed are '\n' and '\r'.
Store the data in an array, and increment an index until the end-of-packet marker arrives. Then, convert the value to a float, reset the index and reset the array.
No need to wait between characters. Many iterations of loop() may happen before the end of the string arrives.
but dtostrf() needs parameters like the length of the char-array
There is a maximum value that you will be sending, isn't there? I mean that 3432412626262.7 is probably out of range. So, figure out a reasonable number of digits before the decimal point and a reasonable number for after the decimal point. Add them up, and add 3 - for the sign, the decimal point, and the terminating NULL. You have the length of the array then.
Thought about that too, but I would need to implement 3 times the while (mySerial.available() >0) (see my YUN-sketch), then each time read the received data and then each time check wether its the "start", the float or the "end" (or non) ... That must be a lot more unreliable than what I already got ...and take longer. So I forgot about that idea
Three Serial.print() statements, or one, makes no difference to the receiving code. It can't tell whether the 12 characters it gets are from one Serial.print() statement or from 12 of them. So, unforget that idea.