Welcome to the club :)
With the car at that age you'd probably be best swapping out not only the springs but also the dampers as they're probably on their way.
There was a group buy going for the adjustable HSD coilovers recently - there were two options, the HR which are the sportier ride and the Smoothies which have a softer ride closer to stock.
There are also these for sale over on Club Aristo - uprated TTE suspension with shocks and springs, although these aren't adjustable:
http://clubaristo.net/forums/showthread.php?t=20824
They're the same fitment for your car, the JZS160 :)
I have the HSD HR on mine, it's the same chassis as yours, just I've got a couple of turbos added :D I'm more than happy with the ride, the only thing I would say is that the HSD suspension is noisy compared to stock, so if you want to keep the car quiet you may be better off with the TTE one