2001 Chevy Impala

I have a 2001 Chevy Impala, V6, 3.8L. I have just recently replaced the radiator, thermostat, and water pump due to a crack in the radiator and overheating. Prior to me replacing all of this, along with the intake manifold gasket (it was leaking and there was a campaign that my car fell into), my car had started to lose power. It doesn't accelerate the way it should and sometimes sounds like it is "sucking air" and it "stutters" or gets "jerky" when accelerating, but stops when I let off the gas. It is throwing a code (P0300) and I have been told that I probably need to replace the catalytic converter, due to the possiblity that it is clogged from burned coolant and that is probably what is causing the problem per the code, but that after that I may still also need a tune-up. I have spent quite a bit of money fixing all of this so far, and I hate to say it, but I am a girl and I just wanted someone else's opinion to see if this all sounds correct so that I don't have to put out any money that is unneccessary.