Amplification won't be the issue. Not too many 5.1 AVRs are geared towards 9.1 signal processing and that's where the hassle will lie. I think the factor for you will be HOW much later you plan to upgrade.
Signal processing features seem to change yearly, and if you hold off for a few years, you may just as well want to buy a new AVR to stay on top of the latest CODECs, room correction, etc.
If your plans to expand are in the near future, say in the next couple of years, then I think you'll be better off getting that 9.1 AVR and add speakers, otherwise, buy a 5.1 AVR and replace it with a 9.1 a few years later.
As for AVRs, the faves around here are Denon, Yamaha, and Marantz.
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