Of the systems you have mentioned only the Ouverture got masterling, and can therefore be linked directly (without any adapters) to your Avant.
I started with the table top ling rectangular system (beocenter 9000), and then got booth the stack (beosystem 5000 (or 5500 and so on)) and an Ouverture. I love them all, but they are all quite different. The beocenter got the best display, the beosystem 5000 sounds the best and can play loud, and the ouverture got all the right party tricks.
The most important question is what speakers you are planning on using. If you got some decent passive speakers only the beocenter 9000 and later and stack system got an amp. The Ouverture needs active beolabs (or an expernal amp and passive speakers as I use).
Alternatively you can play the music of the Ouverture through the speakers of the Avant, it should sound quite well. Then when your budget increases you can buy some beolabs for the Ouverture. This is probably the best long term solution, the stack systems and the beocenter 9000 are getting quite old and the ouverture is not that expensive used. I got my ouverture for £400.
happy hunting
Christian