Quote from CronosSo you can basically just replace the default sounds with new ones?
If so that's pretty awesome.
kind of, MIDI keyboards don't usually come with the built-in speakers, sound drivers or synth libraries that drive up the cost of digital pianos. it's just a MIDI output that you route into a VSTi on your computer and the audio goes through your PC's audio output.
since you're on a budget it'd make more sense to pay for a $300 MIDI keyboard and download a grand piano VSTi. the sound quality and keyboard construction will be way higher than what you'll get for a comparable digital piano, especially in that price range. you can definitely get a nice digital piano for under $500 but I'd suggest the MIDI option, especially since you can get any sound you'd like out of virtual synths.
Quote from CronosAnd wouldn't velocity samples be somewhat restricted by the particular keyboard?
yep. the number of samples isn't really important for your intentions, though, as long as it's velocity-sensitive it should be fine.