I'm guessing you looked on realoem.com or some similar website for those part numbers? Anyways, if you want a factory alarm, you will also need to do some coding (it is most likely disabled in the ECU), plus you might also need a different rear view mirror (you probably do not have the LED right now) and I'm guessing some more parts (I find it hard to believe that the "Altern.power siren with inclination indicat" and it's bracket are the only two parts you would need.
If you go for the factory alarm, I suggest you order and have it installed by a BMW authorized service station.. I wouldn't do it myself, it's quite easy to mess up the system of the car.
A bit off topic, though.. when I read your post, my first reaction was "did your CA never informed your family about all the features your family would miss?". It's always important to get informed when buying a new car of what the car has for options and what it doesn't have. The alarm, btw, does not come standard on the 328i but it is standard on the 335i... I'm a bit amazed you didn't know this before you purchased the car.
Of course, you could have all those things you mentioned standard with BMW, but then the starting price would be somewhere around 45,000-50,000 for a 328i, and I doubt many people would have liked that. Sure, one can stress there is not enough standard options on the BMW, at the other hand could you stress that BMW gives you the option to lose a few options that you don't really need and make the car cheaper for you so it's easier to purchase. Think of it like this: if the cheaper car manufacturers would give you all those options, many of their cars would start 10K cheaper as well
