i had the full milltek system on my ST, 4-1 manifold plus de-cat and what a noise! sounded awesome tearing along the road!
best thing to do if your not fussed about the standard system is to cut the standard system down as its all one piece and get a pipe reducer to fit the aftermarket system.
means you can fit the standard cat for MOT time, police will issue you with a ticket if they catch you running a de-cat, will more than likely make you get a garage to check its been chaged and stamp the ticket you get.

the system and K&N is a good start, plenty things you can do to mod the engine! its a strong reliable engine that fords lumped in it!

you could always get a mazda mps engine! positive there identical! plus if i remember right its a 2.3 turbo!