digital piano + laptop definately. Unless you will be carrying your keyboard away from home frequently... The workstation (even the huge ones) are no match for a powerful laptop filled with software.
In terms of price it's something that needs to be discussed at a later time, but we also need a 'glipse' at your budget...
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In order for the one to compose you will need some kind of instrument, possible with some extra midi capabilities (mod wheel comes to mind and expression pedal), but these can be add-ons. You will also need either a sequencer (Cubase, Sonar, Repear, Logic, Pro Tools, etc), and/or a notation software (Finale, Sibelius, Notion), etc depending on the outcome the composer wants to accomplish: If they want to output a score then it's notation, if they want to output mp3s/wav files then it's sequencer. If they want both ... well... then it's both.
For the other one you just need a nice weighted keyboard with or even without some nice piano sounds. If your weighted keyboard, because of the first person, is hooked on a laptop with some nice piano sounds (Ivory, pianoteq, Garritan Steinway, etc), then you don't even need internal sounds in your keyboard...
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Personally, for my own needs I use a Casio Privia PX110 (OLD model...). It has some shortcomings, but it had a very nice touch for a piano player, and since I use it in my studio I did not care for the internal sounds. Problem was that it did not have great midi capabilities (no mod wheel, or pitch bend), and no in/out hooks for studio work and putting it on a console.
All problems have been solved though, so I'm fine with that at the moment... :-/
