Answer to 3. : You'll have to destroy all the resonators before adding new ones.

Answer to 1./2./4. : If you're L1, you'll need a lot of bursters to destroy the portal. A XMP Burtster L1 can make 150 points damage when you stand on the resonator. 

A L1 resonator health is 1000 points, so for each L1 resonator you'll need 6-7 bursters.

A L2 resonator health is 1500 points, so for each L2 resonator you'll need 10 bursters.

A L3 resonator health is 2000 points, so for each L3 resonator you'll need 13-14 bursters.

[source][1]

BUT, if the portal is correctly protected with shields, (4 very rare shields, each one eat 10% of damages), they will **reduce damages by 40%**... so you'll need A LOT of bursters and time to destroy the complete portal... 

And during the attack, owners can restore health of portal... The best solution would be to go there with many friends fully equiped :)

EDIT:
I found an [attack simulator][2] that will help you to predict how many bursters you will need to destroy a portal.


  [1]: http://gaming.stackexchange.com/a/96672/18551
  [2]: http://ipas.graphracer.com/