Good question OP and a very important consideration.
Firstly the standard merc tracking tech certainly on the 205 is not up to standard and if the 204 is similar you can assume you have no immobiliser or alarm if your car has keyless entry.
The easiest is to use other measures like a diskloc or stering lock which is a visible deterrent. the scum either will come in and ask for the keys or saw through the lock or your steering wheel or not attempt a theft.
Next up is the ghost, IMO, great tech and for the cost , is probably better than a tracker. It also has a valet mode which allows the car to be driven without needing the code but if you go over 30mph for more than a few seconds then the next time you try and start the car, it needs the code again.
Last, tracker - the most potent is a CAT 5 which needs a fob / card etc to start the car , and also the police have the ability to remotely immobilize the vehicle. So that's as potent as it gets.
S5 is the next level but doesn't have the police remote immobilization feature
Then S7 (was CAT 6) which is mainly just tracking and battery disconnect alerts. My insurer asked for a tracker so I stuck a CAT 6 in rather than 5 as my view was if they get the car, they can keep it. That said, this logic is flawed because it is possible for a CAT 6 vehicle to be recovered too...