You should really replace the shocks and springs for best results.
I have a 30mm drop on my car, with Eibach springs and Koni gas top adjustable sports shocks. Ride is firm, handles great, looks the dog's proverbials!
Going TOO low will adversely affect the handling and also looks rather silly, and can cause problems if you ever need to load up your car with luggage/passengers! That's why I went for just the 30mm drop.
You need to watch out for scrubbing though with larger wheels AND a drop, check the clearance between the tyres and the arches to make sure they won't catch. Assuming the 18's you chose are of the correct fitment in terms of offset (that is the ET number) and width though it should be ok.
I'd probably get yourself onto a Ford or Mondeo forum though as there'd be people on there who'll have tried and tested suspension set ups who can advise better!