2004 Honda Odyssey

My vehicle has been hard to start lately and surging while idling. I took it into a private repair shop and they diagnosed it as a bad idle air control, which they replaced for $402.52. A couple of hours later they called back and said that I had a bad spark plug and a bad coil. They wanted $356 for this. It sounded a little high to me so I told them to hold off on it. When I picked up my vehicle, they advised me not to drive it very far as this bad spark plug and coil could cause my catalytic converter to melt. Is this true? How do I know for sure that I need a new spark plug and coil? I have no indicator lights lit up on my dash. When I picked up the car, it was running only slightly better. I am notorious for running the vehicle with a low fuel level, so my dad advised me to get some Heat and add to my fuel tank so it would get rid of any water that may be in my fuel tank. It is running a little better now, but I am still worried that there is something else wrong. I don't want to melt my catalytic converter or be left stranded somewhere.