clifford g4/g5 are good stuff but does not come with GSM tracking (I think).
There are a number of other products that offers tracking like Captor.
What she should be looking at would be one with the following features (I assume the Z3 is a convertible):
Required:
1. Shock sensor/microwave/proximity sensor
2. Tilt sensor - to avoid the car being towed
3. Active / passive arming (in case forgot to arm the car, you can program for the doors to lock itself but be careful as sometimes ppl forget they left their key inside the car)
4. Code hopping (to reduce duplication)
Options
5. 2-way remote paging. Remote has status display on car's security (door open and stuff). Range about 100-300m, unobstructed
6. Tracking (GSM-based, satellite-based)
7. Immobilizer (card-based, chip-based, numeric keypad-based).