Hi, Looks like you will need an Arduino Mega, as the Gas Sensors (Taguchi Gas Sensors) have analog outputs. The Mega has 16 analog inputs and lots of Digital I/Os for your other stuff.
Take a look at the ArduinoInfo WIKI here - http://arduinoinfo.info
The MEGA info is here: http://arduino-info.wikispaces.com/MegaQuickRef
You might want to make easier cabling / connections with a Sensor Shield. See http://arduino-info.wikispaces.com/SensorShield
and scroll to bottom. There is cabling info there too. Mega1280 open source copies are about $30
What gases are you looking for??