Designing a simple ELF-VLF Receiver with Loop Antenna
Are loop antennas limited to single frequencies or can they receive
  specific frequency ranges too? How can i design the best loop antenna
  to have the broadest bandwith to fulfill the 3-30KHz range? Should i
  consider anything special?


You can resonant-tune a loop antenna with a capacitor to get high selectivity or you can leave it un-tuned and feed it into a high impedance amplifier. In both cases ensure that the natural resonant frequency of the coil (plus the spurious capacitance between turns) is not resonant anywhere near to your operating frequencies. 

I would suggest that you use decent bandpass filters after your 1st stage of amplification/buffering to target any frequency ranges you think are useful.


  How can i match the impedance between Antenna, the coax cable and the
  receiver?


Don't bother with matching - you won't get standing waves at 30kHz or below unless your cable is many kilometres long. A short length of coax will not look like a constant 50 ohms for the loop antenna - your amplifier input impedance will be left largely intact and that will be the "loading" on your loop. Please do consider cable capacitance though - this might be enough to cause a resonant peak and disturb your readings rather like a tight bandpass filter would.


  And, how can i match antenna impedance to remove imaginary part?


Your signal won't have an imaginary part unless you are close enough to the meteor for it to dynamically alter the characteristics of the coil!


  What is the best way to limit the maximum output of the board to
  ensure that Sound card of the laptop would not get damaged because of
  extra voltage/current?


Feed your output via a 1k resistor to the sound card input and clamp it with back-to-back diodes is probably overkill - I'd recommend limiting the final output amplifier to not producing anything more than (say) 2Vp-p either by potential divider or by restricting the final amplifier's power rails.



The design of your loop antenna is just a simple magnetic field detector - you should not bother trying to detect the E-field because your loop would need to be about 1 km in size.