to replace shocks or not......
as a mechanic with a fair bit of experience lowering cars, my suggestion is to spend the extra $$ and replace the shocks as well.
firstly, if your shocks have any significant amount of k's on them already they will most likely begin to leak soon after lowering the car. this is caused by the seals on the shaft being moved from the position they were originally bedded into to another point on the shaft.
also, when you reduce the ride hight of a vehicle you reduce the amount of suspention travel where the shock is working. shocks matched to the hight of the car will be more effective as they are designed to work in this part of the shock travel.
and finally, most major suspention parts makers make 'matched sets' which will improve the ride much more than just fitting lowered springs.
also, camber/castor kits are very important here. when you lower the car you always change these settings, and without the adjustors, tyre wear can be a real problem.