I'm looking all over the forums in order to be sure that I'm able to have two sensors (temperature in my case) and a proximity sensor.
Can anyone guide me to a compatibile sensor (I'd prefer a cheaper one) and some starting points?
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You don't need a PiFace for sensing temperature. A Dallas 18B20 sensor and a pullup resistor can be attached directly to the Pi's GPIO pins.
Raspbian has the necessary drivers for this "1-wire" bus - so you can read the temp from any scripting language (BASH, Perl, Python, etc etc)
Adafruit's tutorial - or this one will get you started. You don't need the "cobbler" (ribbon cable connector) and breadboard, you can just solder the resistor to some wires terminated with individual female header sockets (I bought a ribbon cable with individual terminations, peeled off triplets then cut in half and soldered the DS18B20 to the cut end and soldered the pullup resistor to the cable nearer the Pi end)