I have purchased several aftermarket gauges; a GPS speedometer, a tachometer, ampmeter, voltmeter, fuel gauge and water temp gauge. I managed hack the stock instrument cluster to tap into the fuel, tacho and water temp (after quite a bit of trial and error!). Basically, I used the existing Z3 pots (fuel and water temp), and put them inside of the cheap 52mm gauges from ebay. They work perfectly.
All original instrument warning lights from the Z3 cluster are now re-wired to Lucas warning lights. Quite happy with this, as I haven't lost any functionality (lights/gauges, etc.) from the original setup. The stock cluster has lost 'a little weight' and will sit behind the Kobra dash.
This is what the Z3 cluster looks like after several nights of soldering. It has since been sealed, to avoid any accidental damage.