Considering the fact that you do not need to modify or customize anything , nor do you need any special tools:
The difficulty of the project: 1/10
And for an 07 I have no idea but you could start by checking if you have fog light harnesses behind the lower part of your bumper where a fog light equipped 07 focus would have them.
If you have them It probably means the car is prewired for them, you would just need the fog lights and grill, most likely a "fog light" headlight switch for the dash and the main relay box will also most likely look the same as this 2000 does or almost, so just put your relay in the fog light spot for it. Good designs do not get changed often and my relay box looks good, I would not be surprised that yours is very similar, if not the same, even while being the next model year. Good luck, and do it it is worth it.