First of all I would search through the forum as there is a lot of previous advice which is difficult to distill succinctly.
And I’m no expert on modifying. In fact I’m an advocate of keeping things standard.
I think the 15mm drop on a factory Sport is perfect. Going the extra 15mm to 30mm puts it too low and at risk of underside damage - in my personal opinion and experience.
If your original springs are not broken or corroded badly then consensus is they can be reused as they don’t gradually lose their effect like shocks do.
If you do replace them though then I’ll happily buy your old ones and put my money where my mouth is
If you are on original shocks then they are quite probably long since knackered. Bilstein B4 or Monroe Reflex shocks are not regarded as performance items like B6/B8 but don’t need to be to do an excellent job. They are both well regarded on this forum and avoid the insurance issue.
I recently used Monroe Reflex and H&R (direct rival to Eibach) -30mm on my Sport. The ride was transformed but as mentioned too low for me.
It follows I will also say BMW shocks (Sachs) and springs are also good in my book but they are expensive.
You will not find replacement -15mm springs from anyone but BMW. Which is a great shame. And why I will buy unwanted Sport springs.
But you must also look at other factors which affect handling if you want to tighten up the ride and frankly make the car safe. In particular a rear end refresh as done many times by the Zedshed on here. The need for new rear beam bushes are an important factor too on most of our cars now.
sits nice too 
has to be my favourite Z on here
