Well you will here the advice that is generally best to buy the best you possibly can the first time even if it means going a bit past your specified budget as you won't be left wondering and asking the 'should of, could of, would of' thing...
So my advice would be to go with the 805 as that is a far superior unit to the 705 to start with, and spend a little extra time saving up for your external amp in the future. The amp in the 805 is a beast and will be able to power your system just fine until you can pull things together for a beefier external amp...
I think you will be happier in the long run if you get the better AVR to start with and then use it as a pre-pro latter down the road...