thrax

Hi there, I am a new owner of a nice '99 mazda protege with 75,000 miles on it, that I happened to acquire just before it needed some fairly major work done. It has the check engine light on and shows error codes p0420 and another I can't remember offhand but which according to a local mechanic indicates the timing belt needs to be replaced. According to said mechanic, p0420 indicates that it also needs the front catalytic converter replaced as well. I have heard that this code may actually be caused by a malfunctioning O2 sensor or a damaged muffler and may not need the cat replaced. I also know that according to the 99 protege manual, these cars typically need to replace the timing belt and cat converter around 70-80,000 miles, so I am seeking advice as to whether the p0420 code may indicate a more benign repair or if the codes do in fact indicate that the car is in need of these replacement parts. I am fairly sure I need a new timing belt because the car has a rough idle, rattles and shakes at medium to high speeds and hiccups accelerating between 45 and 50 mph. The mechanic offered to replace the timing belt and water pump and some other belts or hoses in the area (not sure which) for 600$ (part and labor). Is 600$ a fair or average price? Would it be cheaper and easier to get both the timing belt and catalytic converter repaired at the same time? If you have any advice it is greatly appreciated! Thank you for your time, I know I wrote quite a bit.