I am also a happy owner of the AV9000 system and I use it in a surround set up. I have Beolab Penta as fronts and CX100 with a ML/MCL converter for the rear.
Sound wise I thnk this is a very good set up for the money. The Pentas are in my opinion superior to the Beolab 8000, and the fronts are where the sound quality is most obvious, I also use the Pentas with the Beosystem 7000 linked to the AV9000 for music in 2 channels.
The VHS-system is pretty useless now days once you are used to digital broadcasting quality, I never use the VHS. I have a NAD DVD for movies. But you have to remember also that the pro logic system is very bad compared to the more modern 5.1 systems. The surrond feeling in a pro logic system is not "for real" as the more modern 5.1 is.
The 6000 and 8000 looks really good! But they are expensive compared to sound quality compared to the pentas (in my opinion). On the rear channels with prologic, you really do not need that "good" speakers. The sound you will have to them is distorted and has a small frequency range. You wil do just fine with just about "any speakers, if they are passive as the cx100 use some of the ML/MCL converters available at ebay for instance.
If the design is what you want and the money not that important, the set up you mention is really great. But for better sound quality use a pair of Pentas in the front and what ever you can find for the rears.
And if money really is not an object go for Beolab5 in the front and Beolab9 in the rear You will have the worst 28" Movie system in the world! But who would seriously spend that kind of money, -if I had them, it would probably be me and no one else in the world...
I am not sure what you mean about the upgrades? It is an old TV so I do not think there will be any updates on that one. All the Beolab speakers will be able to connect, just plug and play. I am sure that there never will be an update for the AV9000 regarding the surrond system, to a 5.1 or 7.1 system. There are of course other ways to get around that if you want. -use the TV as a screen only and use a more modern system for the sound, but that is not so cool as hanging on to the old high tech