springs

Hi, in my daily E36 328i I have H&R's cup kit which has 2.4/4.5k springs (which would translate to 135F/252R lb/in) which is of course really soft. If I wanted to upgrade from these springs, keeping the H&R shocks, what would be the highest spring rate I could go to, without springs overcoming...

Just bought an e30 and need it lowered, anyone got springs laying about or a set of old cheap coileys that will do the job for a while?? Near to Essex as possible