X2 for that. For True coilovers you have to cut out the old turrets and weld in new ones with a plate to accept the top mounts, you'd then have to strengthen the trailing arm to take the added loads from the coilover unit.
Absolutely nothing wrong with shocks and springs on the rear, only problem is getting the height set to what you want, but that would be no worse than getting your spring rate matched in using Aftermarket coilovers designed to fit a different car.
Spax will build you a shock to the dimensions you want and there is a company on here that have all the measurements for the standard springs and will manufacture a set of replacements again, to your spec.
The Coilover sets you can get for the S12 are Just S13 fronts with springs and shocks on the rear, they have an adjustable perch so that you can alter the ride hight +- about 2 inches
__________________
|