Some common questions that come up when buying a exhaust for the Duratec are:
What are the advantages?
Well the stock exhaust diameter on the Duratec exhaust is 2.25, which can be a little restricting on exhaust flow. So a bigger diameter of 2.5 inch will not only allow it to flow more, but add HP too, The HP generated by just a CAI and 2.5 diameter exhaust will improve HP anywhere from 9-13 HP, but it will sound better too depending on your muffler or resonator choice.
Would I want to go 3.0 inch exhaust for more power?
No, not unless you got a turbo or you got a Supercharger, or the only other way is to have all the n/a performance mods available.
Where can I get a full cat-back system?
A lot of places sell cat-back exhaust systems, a few are.
What should I get a exhaust system made of?
There are just a couple of option's for what your exhaust can be made of. one of the most popular exhaust to put on your car is 304 stainless steel.
Mainly because it will not rust like your stock exhaust will.
Also there is aluminum exhaust out there for the focus now, and they are good for weight reduction, other then that they are really not worth it unless your are making big power and need it to be a 3 inch diameter and light weight for autocross or races, things of that sort.
Some people when they buy there cat-back system they choose to have it already having a cat and muffler, and maybe even a resonator. A couple ways of getting a little more sound and HP out of your Muffler or resonator or cat is to get certain ones. If you got a 2.5 inch exhaust with stock header, a good way to make the exhaust flow a little better is to get a high flow cat. Same goes with the muffler, except you will want to go with a Center/center set-up to make it easier for the exhaust to flow right through without restriction. And as for the resonator, I personally only use them to quite the car if the exhaust is too loud, it is a little restrictive to use a resonator, but with the high flow car and muffler, and 2.5 diameter piping... you won't even notice anything but it made it a little quieter.
How this helps you to make a wise exhaust choice.