1 Those Yamaha amplifiers and receivers are excellent (and the heaviest is likely to be the best, because it will have the biggest power supply). They are powerful enough for your efficient speakers in a small room (and would also drive less efficient speakers as long as the room is not really huge). I would suggest you get a Yamaha model with digital inputs, to use a streaming device like a Chromecast Audio, and to connect to the digital output of for example a television. If your budget stretches to 900 euro, why not get a Yamaha RN 803d networked receiver, as it has room equalization built in (plus many other useful features). This will transform the quality of the bass in such a small room.
2 Don’t worry about the impedance, as long as it does not drop too low (5 Ohms is fine for any modern amplifier).
3 Don’t worry about the bi-wiring. This is a fashion unsupported by science.
4 Stereo amplifiers may indeed be a better choice, because you don’t pay for what you will not use, because they do not have to share the power supply with so many channels, and because the inside is less cramped with circuits that may interfere with the standard audio. On the other hand, since they are produced in far larger numbers you can get spectacular deals on them.
Enjoy.
2 Don’t worry about the impedance, as long as it does not drop too low (5 Ohms is fine for any modern amplifier).
3 Don’t worry about the bi-wiring. This is a fashion unsupported by science.
4 Stereo amplifiers may indeed be a better choice, because you don’t pay for what you will not use, because they do not have to share the power supply with so many channels, and because the inside is less cramped with circuits that may interfere with the standard audio. On the other hand, since they are produced in far larger numbers you can get spectacular deals on them.
Enjoy.