A nicely balanced setup is about 3:2 Front-to-rear ratio. You can run the same rates front and rear but in a lightened car this isn't needed.
I can recommend 4kg/3.2kg for road use or about 6kg/4.5kg for an ocassional road/lotsa track car.
Go coilover conversion on the front. I can sell you a brand new pair of struts for conversion if you want. It's an easy cut and weld job and will transform the ride and handling of your car.
Front springs i cant help with at the moment, i might be able to get you some soon however. These would only be lowered TRD Springs however, and possibly dampers but it depends what we do with this particular car and when.
As Will says, coilovers up front are best, but i prefere stiffer spring rates than he does (particularly for the rear). If you lower the rear you'll need a new adjustable pan-hard rod, and if you lower the front i'd recomend you get some RCA's aswell.
I think i can get you shocks, RCA's and the panhard rod also. PM me for details