Necessary or legal? You only "Need" one O2 the O2 between the CAT and the head. you don't need the other O2 or the EGR. However, it is illegal to remove emission devices from a registered street vehicle. That includes the EGR and second O2.
Some shops will do work for you and others may not. I am not sure how the laws work for shops that work on a vehicle but I would think they may have some liability.
Usually people will do a O2 sensor eliminator "hack" this is usually concealed better than completely having no O2 because during a physical inspection any slightly knowledgeable car guy is going to know you need a pre CAT and post CAT O2 sensor and when they see no sensor they are more likely to look deeper than if they an O2 wired up. Same can be said for the EGR. If people know what they are looking for and know the focus engine bay its fairly obvious the moment you open the hood if you removed your EGR or not. However you can always hook it up and just disable it via software also.
I am a person who likes to leave the hardware and its functionality available so in the event something is about to happen I can flash it back to stock and do a few drive cycles and get it inspected and all is well. Then I can flash it back disable a bunch of stuff and be on my way for another year.
That is the same looking CAT that I used on my 4.0L Jeep Cherokee when I was doing my own custom exhaust. the CAT may be to long to attach a flange, bungs, and start the 90* bend to go under the motor. What is the measured length of the CAT? you need 2-3" at minimum to add the flange to mount to your shortie header and about 4-5" at the bottom for the bend. You also have to consider the depth. How deep is the part? I think the stock CAT has the inlet and outlet offset so it would push the CAT further from the motor towards the radiator.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)