I wouldn't go near coilovers if you think the ride quality is bad on oem, of course coilovers will ride the same, or slightly stiffer if not stiffer, especially with the 400 euro price you have.
I'd look at H&R street suspension kits or Koni STRT kits or even the AP kit, consisting of Springs and shocks, no adjustability, but will handle and ride better too. I've experienced cheap coilovers and most do ride slightly harder than stock, depending on wheels, and the amount lowered