1998 Honda Accord Catalytic Converter

Computer problem
1998 Honda Accord 4 cyl Front Wheel Drive Automatic 141K miles

Hi, my catalytic converter has been setting off a P0420 code. My mechanic said it means it has to be replaced. However, the MIL light has been on for 2 years in relation to the catalytic converter. I got an estimate from BB Muffler and it would cost $550.05: $90 for Labor, $65 for Air/Fuel Mix Test, $337.17 for the part, and $27.77 for shop supplies. The reason I've delayed this costly purchase is because I've been attending university and I don't actually feel like there's a big problem. My question is 1. Given this cat has been going strong for 2 years does it really have to be replaced? 2. Am I in any danger of a breakdown such as when a radiator hose bursts and such? 3. I've done some research online and a lot of people are getting their cat converters replaced for $100-$300 but I can't find that here in Austin, TX. Any advice on getting the best value? 4. The part cost online is $39, Magnaflow Catalytic Converter - 91000 Series Universal: SKU: 6691004CA. Would this be a sound alternative and do local mechanics accept parts from customers and charge only Labor?