no problem here in holland with insurance and modding ..
the only thing you cant do is change the engine without getting it sorted at the rdw (sort of big boss mot place)
but even that is easy as hell my m8 has got a corsa a with 2.0 16v and it took about 20 minutes to inspect and its legal now all stated on the (new) papers
but like 50mm drop is quite heavy already imo
and too much for just springs , maxx you want to go lower on just springs is 40mm and that depends on the car , most cars 35mm is doable
50mm springs will need new stiffer shocks to compensate the bouncing and not hitting your wheel
so that said adjustable suspension is the way to go .. would be the same price as springs and shocks and is all in one and can be set to whatever ride hight you want..
also srings are a fixed lowering , so it could be too high or even too low , adjustable shit is perfect set it how you want
only saying this because you have a pretty agressive wheel fitment , with regulair wheels this would be way less of a problem
