With 105K miles, you could have 3 dead cats, but a second opinion would be best before you start buying parts & labor. As mentioned, replace the upper sensors and don't worrry about the lower ones for now. The pre-cats can be gutted if they are really clogged to save $1000 or so. You can get an aftermarket high-flow main cat on the web for under $100 or get a replacement at a muffler shop for about $200 or so. Get some mil eliminators for the second O2 sensors.
