You will always pay too much at the dealer. You may want to search around but here is a catalytic converter at autozone for $80. If you dont have any experience at working on rusted exhaust parts, maybe a muffler shop such as Midas, Mienke or similar would be a good idea. Basically, if you want to do it yourself you may need things such as a torch to heat up the pipe so you can get them off. You could always cut them off if you know where to. A muffler shop can probably replace 1 Catalytic converter in 20 minutes and save you that much in tools. Not to mention to really get a good grip on exhaust the vehicle is always best on a lift. Just trying to get tools and your arms under a car on jack stands makes exhaust work really difficult, although I am not saying it cant be done. I think the most important thing here is the price of the Catalytic converter.
I can suggest checking the exhaust itself for leaks or holes. Put your foot over the exhaust pipe while it is running and listen for hissing. If it hisses you may need someone else to hold there foot over the exhuast while you look and feel. You will usually feel hot air coming out wherever the hissing sound is and maybe even see the hole or why it is leaking. Maybe a rusted hole or loose connection on the pipes, etc.
If they told you the catalytic converter was bad it could be a few reasons why. Maybe it just isn't doing its job anymore, maybe it is throwing a code to the computer, maybe they think that it is clogged up or it is just remaining hot after 30 minutes after shutting down the engine. Did they tell you why you needed a new catalytic converter and if so how many and\or which 1. Do you know how many there are on your vehicle?
You would need a OBDII scan tool to check for codes and they are quite expensive. You could call around your area to auto-parts stores and ask them if they check codes for free, alot of them do.
Thanks for suggestions - I will check that - sounds logical. And yes Autozone checks codes for free but last time I took it to them they said that the interface to the car was shut down or not working. Do you know if dealers are doing that to force patrons back to their service center?