I got this ramp for my 911 as you need to do everything from underneath the car but it is useful for any car under 2 tonnes.
https://www.youtube.com/watch?v=qtjxYkWqrIg
You can skip to about 3:30 if you don't want to know how to adapt it to work with a 911. It should be fine for a BMW Z3 straight out of the box.
Google CJ Autos for spec and prices