hi,
RF modules will do , but they will increase a little the cost of your project and beside that you will need to spend a little time programming them ( i worked with PmodRF1 transceivers in the past, i think i could give some help if so ....).
Of course your first idea with wire would bee cheaper, so it worth to stick around this idea.
my suggestions :
-You can use cable to take the analog signal from sensor to pic mcu, ~40 ohms is not so much, but you have to consider that resistance is modified by temperature , if that does not affect to much your measurements , it's ok.
- instead of transmitting voltage signal thru cable you could transmit a frequency signal witch is proportional with the output voltage of the sensor. To do that you need to use a simple circuit to convert the voltage from the output of the sensor to frequency (i may have souch a circuit schematic using a 555 and 741 opamp, that i built in the past... if needed ) .Then you use the the pic mcu at the other end of the cable to process that signal . You could also build the voltage to frequency converter with a small PIC that has ADC ....
- ....or you could use 2 PIC's , both with UART (or USART) , and one with ADC . You just have to google about maximum length of serial cable
In principal as i see , your problem is about information transfer over 40 m distance , you can solve it easy in one way or other ....