Hi mate and WELCOME TO THE FORUM :wave:
Lots of people fit the focus induction kit. I'm assuming yours is a 1.2 8v?
Its legal and it will pass an MOT. However it would need to be declared to your insurance company and may increase your premium.
It depends what you want it for as to weather its worth it or not. If your looking for performance gains, they are hardly going to be noticeable in the grand scheme of things. If your looking for noise, then the focus kit normally produces the sound that people are after.
Hope that helps,
Alan