At this point, you will have to make a decision on whether you are chasing the latest digital formats / standards or if you are chasing sound quality. The two do not always go hand-in-hand. The HDMI 2.0b standard for HDR is so new that hardly anybody has implemented it. You will always be behind the curve within a year of getting a device because they will always be bring new technology to market so that people will "buy more".
The Yamaha is a good choice for that price point, but I would pick the Arcam AVR 850 over the Yamaha unless the price was an issue. I believe the Arcam will just have a more refined sound (even if used as just a preamp).
There could be a benefit of getting a 2-channel amp over a 3-channel amp in some cases. If the manufacturer used the same power supply in both the 2-channel and 3-channel models, the 3-channel would have less power distributed for each channel. Some manufacturers will increase power supply for the 3-channel model. Some will not. In the case of the MC303, I would not worry because McIntosh has definitely increased the power supply for 3-channel. The watts/specs also match. Look
MC302 - 2 channel - 98 lbs
MC303 - 3 channel - 155 lbs
There is one benefit of getting a 3-channel amplifier and that is that the sonic signature for left/center/right speakers would match exactly. If you get a 2-channel, then the center channel speaker would have to be powered by the AVR, which could/would have a different sound. This would be noticeable when sounds in a movie pan from left to right (such as a motorcycle driving across the screen).