E30 cabrios rattle like a mofo, they lack quite a bit of structural rigidity. If it's a sporting car you're looking for, getting a drop-top is like putting on boots for a marathon. It's heavier, weaker, and to add to the pain, needs specific suspension parts. And the rear window is plastic.
Then again, there is nothing quite like the open air. It's a tradeoff, choose which way you swing.