What are the standard wheel sizes? If you fit low profile tyres you might be able to keep the same overall dimension and preserve handling and not have to mess around with getting the speedo recalibrated. Stiffer springs will help a little but lowering on 18's can make the handling worse on larger wheel sizes. Personally I would go with 17's as they are cheaper unless that is what you have got on there at teh moment and then I would lower the car 30mm to help fill the arches. This should save problems of grounding, tramlining and bumping.
17s are also much cheaper to get and tyres are cheaper too!
Welcome to the site and it is nice to have you along.