Well 'too low' is more a personal and only you can answer that.
With the right mods any sax can go down as low as 80mm even more but you have to work out where you can go in a car that low.
40mm is about as low as I would recommend people to go as any lower and your asking for trouble with exhaust clearance problems and depending on what rims you running clearance problems there too.
Look on eBay and on
www.performancecentre.co.uk and
www.motorsportworld.co.uk for good prices on just springs.
Mines fitted with Pi kit and is ment to be 35mm but is probally closer to 38mm pushing 40mm now its settled down
Any lower and the tyres would be too far in the rear arches fo my liking. But like I said personal taste and different people will say different drops.
Hope that helps