Heya mate, I have just done mine and I have to recommend Eibach, yes they are more expensive but they are brilliant!
I personally would not go past 35mm as I was talking to a guy off of here and he had lowered his 40mm on G-Max springs, he ended up failing his MOT because the shocks couldn't cope with 40mm on the back (it was something like that anyway)
If you are lowering for looks then I'd say drop it 35mm on Apex springs, if you want looks and handling benefits then go for the Eibach 30mm Pro Kit springs, I have them on mine now and they are brilliant, I've kept the ride quality pretty much the same also because of Eibachs use of progressive springs, no other manufacturer use this so I think it is just an Eibach thing, just means that if you are not leaning on the springs then they go a bit softer, but when you lean on the springs in a corner they firm up, meaning you hardly notice any body roll. (Not an official explanation I know
)(Btw, I don't work for Eibach
... but if you decide to buy them then I recommend you contact MeisterR from this site, thanks to him I ordered the springs Monday at 7-30 PM and received them Wednesday afternoon. (Btw, I think they are £180 delivered)
On the wheel size, mc183rufc has dropped his 30mm on 17" wheels and his Coupe looks brilliant.