Hey, welcome to the club, LS cars cost the fortune outside the US for some reason:)
Transmission problems are not common for LS, they were built to last forever. howewer, leaking power steerting system is really common and not fixing it will lead to the dead alternator so this is a good thing that you fixed it. suspension is also pretty durable except the bushings which might require change at the age of your car.
You are right about the radio, you cannot use the amp if you change the headunit, you will not be able to use CD changer as well so you will have to change evertyhing, you might keep the speakers but considering their age if you will change whole system you better change them as well.
I really dont know about the trunk safety feature, i can open mine even when i'm driving:)
Good luck and let us know if you have more questions.