Keyboard MIDI controller hooked up to DAW/cheap synth might make it easier playing things like chords & certain riffs into the MPC for sampling. Would help towards learning music theory & playing an instrument. With a stand-alone synth, effects, & some go-to drums you always keep around, you can step away from the computer too. I find that even with the MPC as my main sequencer & MIDI interface, I still like a 61 key controller set up with it so I can play certain things in. I'm pretty sure everyone here at one point will eventually tell you to get at least one nice synth you gel with, but it shouldn't be necessary for awhile; it's not really about playing something, but using it to fill the gaps when the samples can't.
MPC 1000 w/jjOS, Elektron Octratrack, Korg EX-8000 & MS-20 Mini, Behringer Neutron, E-mu E6400 Ultra, circuit bent Roland TR 505, Mackie 1604 VLZ3, Electrix Filter Factory, delays & compressors