The BeoMaster 5 can connect via VGA, DVI, HDMI, Composite Video, or Component Video. I don't quite understand what advice your dealer gave you, but I'm sure you have at least one of those to spare on your BeoVision 6. The BeoMaster 5 is certainly usable without the head unit, but I would suggest STRONGLY, that if you're on the fence about whether to buy the complete package or just the BeoMaster 5 that you at least audition the full system in your own home with your own music. If you're already buying the BeoMaster 5, your dealer should have no problem lending you a unit for a few days once your music is loaded.
As for the main question...
As for the BeoPort, you have to understand that they are very different products. One streams content with very limited control and one give you greater access than you have on your desktop. If you are a casual listener to your digital music collection, you may as well save a little money and purchase the BeoPort (assuming you have a BeoSound audio system, otherwise it won't work).
If you're like me, however, and possess a very serious music collection (over 850 albums so far) on your computer and that music figures heavily into every aspect of your life (as it does mine), you won't regret a BeoSound 5. The way it puts you in touch with the music is like nothing else out there. The sound quality is astonishing even with most of my music at 256 or 320kbps and it's just sexy as hell.
Sonos is very interesting, but really not a serious contender. Poor sound quality and very limited integration potential make it ideal for a casual music listener, but a novelty for the serious music listener.
Hopefully this helps a bit. It sounds like there must have been a miscommunication between you and your local dealer.
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