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Its fairly easy and cheep to replace springs, its not like changing torsion bars. You can always go softer or harder later.
I got 400/600 in my track car, its not very stiff for a track car, but you probably want softer for the streets.
My street Porsche 924 (50/50 weight) has somewhere 325/400.

The 250/350 suggested above by Doc is probably pretty nice for the street. I would probably go a little stiffer.

Harshness is mostly in the shocks and bushings. With good shocks you can use pretty stiff springs.