From my experience I would say that youre better off getting shocks at the same time. If you get a full kit then the springs and shocks are going to be matched to each other and work well together. If you keep the standard shocks then theyre going to wear out pretty quickly and youll end up paying out again for them to be replaced, and the ride will be very harsh.
If you get the suspension fitted at the same place that you buy them from you might get a discount on the fitting, however it does depend on who you get them from.
Rick