Great advice. I know this is very much a specialist area and one can spend a great deal on getting a rally set up. As people say, it's a process of experimentation to see what works. The weight of the car at each corner, getting the correct rate and length springs (what about variable rate?), and adjusting the dampners to match (non gas being adjustable with different oil weights).
I have a CA18 with 200b struts, so I would need front springs 50% higher rate than the stock 1200 in theory (A12=87kg; CA18=128kg). So stock spring rates are 91 pounds/inch, one would need a 47% increase in rate which equates to around 133 pounds/inch. Don't know if that really holds up in the real world though! Are the advantage of custom coilovers simply to offer more -ve camber?

Sounds like keeping the stock leafs rear (people have used a single leaf with good results apparently) with good shocks is adequate for a budget set up which is probably what I'm after.