Also looking at some suspension upgrades myself at the moment and there is a decent amount to consider when making a choice. I think it's also good to consider not just suspension but also the wheel and tyre combinations that you want to run. By changing parameters with ride height and wheel sizes, the comfort to perormance equilibirum will shift.
Heres some of my ebay watch list:
Fixed position:
Bilstein b4 kit -
https://www.ebay.co.uk/itm/4X-BILST...e=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649
https://www.ebay.co.uk/itm/Eibach-P...adster&hash=item417d412616:g:~8cAAOSw631c3UQD
Bilsein B12 Kit
https://www.ebay.de/itm/EIBACH-BILS...ke:BMW&hash=item56c91493b3:g:jJIAAOSwWAldUvQY
As a lot of forum members have said, these are a tried and tested, happy formula for a lot of Z owners. Probably the cheapest option if its just springs. The shocks might still have some life left.
Coilovers:
https://lowerks.com/vogtland-coilovers-bmw-z3-95.html A set of Vogtland coilovers will give you 30mm drop at a minimum with more adjustabilty in ride height. When buying coilovers, its worth looking for some trademark symbols, TUV, which is the german road standards agency have strict vehicle codes and cars must used tested parts. These are about as cheap a good quality coilover as i can source. Also TUV rated.
If you are feeling flush
https://www.driftworks.com/hsd-coilovers-for-bmw-z3-96-02.html HSD do coilovers with top camber plates too. And for a deeper dredge of your pockets bc racing, kw and up will be the peak in terms of coilover options. These places like driftworks and demon tweeks also seem to offer some interest free credit for 6 months and i would assume other retailers too.
There are some cheaper coilovers on ebay available but i have never heard of anyone using them so could not report anything on those but there is a phrase, you get what you pay for and i think its worth bearing in mind.
Wheels&tyres
If you intend to run 18-19 inch wheels more often, i would say look at a Bilstein B6 strut with a standard spring for 15mm lowering, alot of arch gap can be filled with wheel and tyre combo and with a reasonable amount of comfort retained. 30mm lowering with 18-19 inch wheels is a pretty rough ride on UK roads and not a pleasant driving experience imho.
The reason i like 17's is that i can have a little bit more tyre sidewall to soften up the ride. There really is a big difference from filling a lost inch of wheel rim and gaining some more rubber when the car is lowered 30mm.