A good triton will be more than a $1500 mistake
I've only used an MPC2000XLSE so it's all I can attest to -
Honestly, those are all different things you're mentioning. The triton is a workstation, all in one sorta thing. It's a sampler, synth, sequencer, etc. The motif is pretty much the same thing as far as I know, then you have the MPC. The mpc is used for midi production, sample editing, sequencing, sampled drums, and some other misc. sampling. It doesnt have a synthesizer, and the MPC2000 doesnt have stock effects but its generally the brains of your setup. It's what is going to communicate to the rest of your hardware to tell it what to do via midi.
The MPC2000 is not a full fledged sampler. You're not going to load 700 different vocal,string,etc samples into it to play at once like you would with, say, a nord lead 3. It's pretty limited as far as lengthy samples go, supporting only 32 megs of memory (which if I remember correctly, translates to 2 minutes of stereo @ 44.1khz, or 4 minutes of mono). But for drums, it's fscking excellent. The sequencer is pretty good and the menu system is straight forward. Yes, you can plug in a keyboard and trigger the drums. In fact this is generally better than using the built in pads because they are uber sensitive and often produce unwanted results.
In short the mpc is REALLY good to use as a sampled drum machine, generally really good for a master midi controller, really bad if you want time dependent effects like delay/echo/slicer, or filters like cutoff and resonance and dont want to use an external device (onboard filtering sucks butt), and really bad if you want to have entire track vocals or a bunch of length samples stored to play back. When my friends come over to hang out, the MPC is generally what they get stuck on. Its really fun, and I think that means more than if you had a boring piece of something better.
my 2 cents
bk