As I've always said.. you've got a german car (never mind the "vauxhall" brand.. it's german-engineered and designed)
So, go for Blaupunkt stuff. It's the best match for german cars! And they have special-fit speakers for Opels, VWs and Mercedes... Get a decent four-way amp, some good factory replacement speakers and, if you're not an audiophile, and it all gets properly installed, it will be the last thing you'll need (of course with the adition of an Empeg).
Will your next car be a german engineered too? or you'll go for something different? anyway, good luck and feel free to ask about Blau Stuff