You are right about the CDJ-900/2000 being the players that you can link via RekordBox Link and play tracks from it.
The mixer can be important for some of us wanting to use spot on (accurate timed) audio effects (by using a feature called QUANTIZE) or use for example traktor (the djm-900 acts like a soundcard interface so you don't need another one which was the case with previous (pioneer) mixers)
The DJM-2000 for example also allows to drop a track onto the mixer via Rekordbox to preview CUE that track before even loading it onto your deck. (quite handy sometimes)
Also, the DJM-2000 acts like a kind of 'router' allowing you to connect two computers running Rekordbox and up to 4 CDJ players via CAT Cables.
but just to clarify, you can just use a switch and a link cable(CAT). you do NOT need a 900/2000 mixer for the Pro DJ Link to work.
So if you can live with manual effects adjustments (automatic bpm detection, not 100 % accurate sometimes), no mixer preview, no dedicated hardware to work with Traktor perfectly (djm-900) and the routing feature of the 2000, then I guess you could also go for another mixer..
I had a Allen and Heath Xone 92 mixer before upgrading to my Pioneer DJM-2000 and while I liked the solid feel of the AH over the 2000 I never used it again. The 900/2000 offer some great options, especially when used with the 900/2000s
You could consider purchasing a DJM-700, DJM-800 or DJM-850 if you don't need the things stated above.
Just remember that the DJM-700 and DJM-850 do not have Digital Inputs.
I do use them as I am an audiophile quality freak when it comes to music ;)
and your comment about the behringer mixer.. if you are really serious about this then don't spend this amount of money to buy a behringer mixer ;) it doesn't have to be to like really expensive.. but it's one of your most essential things for DJing.