The DJM-850 has DVS support for rekordbox DJ. I also wouldn't get rid of your Serato box just yet. I would suggest giving the the trial version of rekordbox DJ a test when you get your DJM-850 or DJM-900nxs and see if it meets your standards. The DVS feature is part of the trial.
Like you, I liked rekordbox DJ for it's organization features, but once I tried the trail version of rekordbox DJ with certified DJM mixer. I found that the DVS currently does not perform as well as Serato with the setup I plan on using.