AC contactor 1 pole or 2?
The L1 and L2 designates the two feeds. The contactor is a one pole contactor with a shunt.  The voltage rating is 277volts.  The FLA designates full load amps, 25 Amps, which is the max rating for the contactor with inductive load.  The LRA designates locked rotor amps, 150 amps, which is the max rating for motor startup, and the res amps, 35amps, is the max amp rating for resistive load like lighting. The coil voltage is 24v and is fed from a transformer in the air handler. Your compressor would be considered inductive load. 

You don't have to go with Siemens but get a good one... remember, you get what you pay for. 

Also, your fan in the compressor, should not be running all the time, only when the compressor is running. Good luck.