2003 Mazda MPV Catalytic Converter

My question is how hard is it to change these on this type of vehicle? I recently took my 2003 Mazda MPV to a local repair shop that I wasn't to damn happy with. first my engine light came on the day before so I hooked up my scanner to see what type of codes I was getting, which turned out to be a P0171 & P0174. It said that my bank 1 and bank 2 were running too lean and that cyl#1 was misfiring. That is when I decided to take it to the shop to get a better diagnosis, my first thought was it needed a tune-up, which it has not had yet. Well needless to say these clowns said they started to run the diag. and said they couldn't go any further and without calling me for my authorization decided to start taking my van apart. Only after I called them back the next day in the afternoon asking if they checked out my vehicle did they say oh yeah it will need two new catalytic converters and an O2 sensor to start with for $2900.00 that's not including the spark plugs that it would probably need.

Please help
jknic