The worlds your oyster mate, depends on how low you want o go and how much you want to spend.
Approx £500 will get you adjustable coilovers and go as low as you want.
Approx £130 will get you a decent set of springs with about a 35-40mm drop.
Eibach springs are good quality, but there are loads of different brands out there. I would go with a name that has been around for a while, Eibach, Spax, PI, Koni etc
I'd fit 35mm dropped springs, with shocks if you can stretch that far, and you will get your 17's in there and it will look good. If your standard shocks are not worn out then you would be able to keep them with the new springs.
Everyone has their own favourite way to go. See what some of the others on here suggest.
Get a copy of Total Vauxhall mag, always loads of ads for springs etc, or use some of the companies listed on the forum, that pepole have recommended.