Lowering my VY Sedan

I'd like to lower my Series 2 VY Commodore, it is currently on FE2 suspension and I'd like it to sit maybe, 25-35mm lower. Just after some opinions on how low I should go and what springs and what not to go with. And how much it will end up costing me.

Thanks in advance for any help.
Here's a picture of how it sits now.

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