For the Apple TV, your experience years ago is irrelevant today. These days, all you need to do is install Infuse on your Apple TV. Infuse will play the native files. It's a complete waste of time to transcode the files.The Core 2 Duos don't have quicksync. Which may turn out to be important.
At one time, I had a Core 2 Duo imac (early 2009, 20 inch). I also had a appleTV, connected to my HDTV. Could I use the appleTV to watch the HDTV recordings on my iMac, given that the AppleTV likes mpeg4, not mpeg2?
Not a chance. Well, if I put in more memory, and waited a day for the clips to transcode.
On a mac with Quicksync, it's realtime, or better. You can even share a screen over wifi.
Of course, now that it's 2017, my imac has a better screen than my tv, and streaming has mostly replaced off air recording.
But still. Things that apple takes for granted nowadays aren't there in the 2009 imacs.
Also, the 9400 graphics chipset is thoroughly obsolete. USB3 is actually quite nice to have for connecting drives. And, a whole host of other gotchas await.
Intel Quick Sync is not necessary in these models for video playback either, because they come with discrete nVidia GPUs, that support h.264 decode in hardware. Anything from the nVidia 9400M or later is perfectly fine unless you want to game or something like that. That means that they can play Safari video and Netflix HD just fine, with low CPU usage.
Now, you will need Intel Quick Sync for hardware HEVC, including 10-bit 4K HEVC, but to get that support, you actually need a 2017 model. Essentially, the ideal video support cutoffs are 2009 and 2017. Hence, I bought Macs in 2009 and 2017.