I'll answer generally with two points:
1. Seek out your nearest retailer even if it's hundreds of kilometers away. A great B&O system is really dependent on a great relationship with your retailer. We're all here to help, but only your retailer can write the sale and he or she will be able to advocate for you, guide you in your decisions, and often actively find you the best pricing so that you can build your ideal system.
2. The majority of your items can be used in any new system once the TV is upgraded. Whether or not a new Audio Master (a current or semi-current audio system that is capable of interacting on the ML bus with the correct option programming) is needed is debatable. Since so much current music is accessed through attached video peripherals (like TVIX, AppleTV, etc.) you can pull music into other zones and use the TV as central control. Just remember that ML is an analog bus, so the sources have to be able to output analog in parallel with HDMI. If your parents are real music enthusiasts, the BeoSound 5 can't be beat and the new Beo6 remote allows browsing and navigation of your BeoSound 5's music collection in any link room.
So if you use your current speakers and simply upgrade your parents' TV & audio system you can more or less continue to use the system as-is but with all the current media sources and the ability to grow for another long period of time.
There is scarcely anything in this world that some man cannot make a little worse and sell a little more cheaply. The person who buys on price alone is this man's lawful prey. - John Ruskin