First thing. Dolby Atmos and DTS:X surround formats are fully backward compatible. That means that you can play an Atmos movie on a system that does not support Atmos and the movie/sound will play just fine.
Second, the newer 4K processors are only good if you have a 4K source. Some of the streaming sources will have 4K. So it's up to you as to whether or not you really want to support 4K. Some of the newer processors may have some additional video processing chips that could help. I'm not sure how much.
If you want both 4K and Atmos support, probably the best processor for the money is Marantz AV7703 (or higher). Marantz is an excellent processor with discrete analog stages and great power supply, but it is voiced just slightly on the warm side with rolled off high frequencies. You should be able to get one for under $2k no problem. They are on ebay all the time (AV7703 / AV7704 / AV7705). They support the latest HDMI 2.0 and HDCP 2.2 standards.
The AVM 60 is going to have much more high frequency response and could sound a lot more exciting, but it could come across a little bright and I don't think it will have as much fullness and body as the Marantz.
If you want to remove your 4K or atmos requirements, there are a lot of options. Such as Classe SSP800 or an Anthem D2v if you can find one. Heck, even the $1k Outlaw Audio 976 processor is not bad (I've tested it).
The Nuforce AVP-18 is really a bare bones processor (switching power supply and SMD op amps for output). There's hardly anything inside and you have a LOT of options for a significantly better processor.
