The Z3 is essentially a E36 (3 series) compact that has been shortened and equipped with a sproty chassis. The rear axle design dates back to the E30 (even older 3-series). This way of reusing earlier sucesfull designs was probably a way for BMW to keep manufacturing costs down. For the Z3 owner, this is a big advantage, since parts (new but especially used) parts are readily available and prices are *generally* low.
The wonderful thing about any BMW apart from being a quality piece of kit, is that it is possible for ordinary hobbyists to get access to exploded view diagrams and part numbers of all the components that make up a BMW vehicle. The first link you need to know about is www.realoem.com - Do yourself the favour of bookmarking it right away - It will save you (and expecially your parts supplier) a ton of time. One of the most useful features is the ability to cross-reference parts between cars - That way you can see excately which "donor vehicle" to look for if you need something: For instance, you might want to see what the bigggest brakes that will fit on your car is. Since the brake carriers are attached to the kingpin, you can look up which cars share the kingpin your vehicle has. After that you look at the largest car (which should have the largest brakes) and chances are that you would be able to do a transplant - Of course you need to check if your brake master cylinder is large enough and if the discs will fit behind the rims etc.
Other useful tools are thw BMW "Technical Information System" CD and the BMW Electronic Parts Catalog (ETK) DVD - The first is a database of technical articles, specifications, service and repair procedures which can be a great help in reparing and servicing your car. The secons is equivalent to what realoem offers online - Just if you would like an offline version. I bought the bentley publishers Z3 service manual, which i have mixed feelings about. It seems a little basic and superficial - however, it is good if you need information on the electrical connectors and location of components.