Too many E46 are on the 3rd-5th owner at this point and many cars only show up with a single key.
Your situation is very common, only 1 key, either with broken door cylinder linkage or a weak or dead keyfob.
What usually happens is people lock their only key in the trunk, loose their only key, the keyfob battery fails or the GM5 module acts up.
Then they get locked out the car and the E46 and other BMW's are very hard to get into with the central locking system.
Sometimes you can replace the keyfob battery and get going but if you are a butcher, you can damage the EWS circuitry and then the engine will not start. But in any case if you loose your only key and your door cylinder linkage is broken, even with a new key, it will not unlock the car with the keyfob as it is not programmed.
If the car has fold down rear seats you can usually figure a way to get the seat down after getting into the trunk.
Moral of the story, do not use the drivers door lock cylinder on a daily basis, however, check it from time to time and have at least 2 working keyfobs.
If your GM5 module acts up, fix it right away.
See my signature for GM5/Doorlock info and for other useful info.
Good luck with your new to you E46.