Subaru Repair: catalytic converter, vacuum leaks, catalytic convertors

Question
My check engine light came on my 2000 Legacy L wagon, I ran the codes, and P0420 came on.  After checking an on line service manual it said to check to make sure there were no vacuum leaks around the catalytic converter.  I need to clear this code to pass my states emission test.  If there are no obvious leaks, how hard is something like this to replace?  Do I buy the catalytic converter one section at a time? (front and back).  Do I have to replace the O2 sensors when I change it out?  I found some  CC's on the internet selling for as low as 90 dollars and as high as 350 dollars.  Whats the difference?  Thanks in advance.

Answer
You should replace both front and rear catalytic convertors. You do not need to change the O2 sensors because they are working fine. If they weren't, you wouldn't get this code. When it comes to these cars, I would recommend a direct fit kit. It's pricier but it is guaranteed to fit and work. The cheap ones either need modification or they are made so poorly that they fail relatively quickly. Once they are replaced you will have to have the codes cleared and then you need to drive about 50 miles before you bring it in for your state inspection. That way the computer will have time to run the OBD-II monitors(self-tests).