I'm guessing by your user name you are a guitarist?
I am, primarily. For me keyboards are only really used for atmosphere/vibe and occasional flourishes etc so I guess it depends a lot on how you want to use them. It might be important e.g. to have a very realistic piano or organ sound? Most keyboards do a fair job but if you are really fussy (I'm not) you'll probably want to pay big bucks.
Personally, as many sounds as I can get (say 500+?) together with a reasonable memory, min. 61 (maybe 76?) velocity sensitive keys, MIDI in/out and last but not least, a pitch bend wheel, would all be on my check list.
That Casio CTK 900 looks OK and within budget - but doesn't seem to have pitch bend/wheel - but of course depends on what you want/feel you need.