Using analog temperature sensor (LM35) and 10-bit ADC MCU for a wide range of temperatures is not quite good combination. The LM35 provide only 0 to 1V to MCU ADC which implies many troubles... Much better choice is digital temp. sensor, as commonly used DS18B20 which have 12-bit ADC and returns digital data to MCU.
Control 7-segment displays with Arduino (i.e. ATmega328p/8 or similar small MCUs) is trivial and there is many open-source projects around.
Using long cable (80m) is a real problem. Notice that any wire, no matter how quality is, have its resistance and capacity - that means you need to count on quality and speed reducing. As well it must be shielded to minimize external noise...
All that means large budget for quite simple project at first look... That is the main reason to carefully specify your project requirements, existed solutions and minimize project budget, no matter it is hobby, student or business project.