I don't use an MPC, but I'm quite sure there is a sequencing function (there's a sequencer on my cheaby Yamaha SU-10!). So that would let you string together a number of samples and play them in a song-type pattern.
The advantage of sampling for someone that doesn't play bass/keys/guitar/drums, etc., or doesn't have the space to have all of those instruments and record them live, is obvious. But it quite plainly let's people who don't have traditional musical know-how or training, but have great ideas and a good knowledge of pre-recorded music be creative with those things and get into music production by sampling and sequencing in the same way a DJ would (except the records aren't spinning live; they're just played back by the machine).
The advantage for the traditional musician or producer is that a good sampler like an MPC is a real electronic percussion instrument-- once you load your samples in, you can literally "play" the heck out of them with the pads, which is both creative and fun. Another advantage is that certain samplers are known for the sound that they have or add to the samples recorded into them. This has to do with sampling rate and various electronic components/innards, but to get a certain type of sound, sometimes a sampler is the way to go.
Since I'm something of an analog/hardware junkie, I might not be the best to tell you which _one_ piece would be best to start on, but certainly an MPC of some kind would be a good buy if you wanted something that would retain re-sale value and that you could use with other gear. But don't forget you'd probably also want some kind of decent turntable for bringing in vinyl, and maybe a mixer, some speakers and headphones.
Other options could be a bunch of separate pieces that could be connected with a sequencer or laptop via MIDI, a laptop with a soft-synth/sampler program that would let you use something like a PadKontrol to get basically the same thing as a sampler and/or keyboard but with digital gear. You could get a stand-alone workstation, or something smaller that might be considered more "toy-like," but would give you a good "in" into the world of hardware that you could also add to your DJ rig if you wanted to (small, portable, but cool items; Korg is excellent at making devices like that!).
I have, among many other pieces of gear, a Roland Juno-G keyboard workstation, a small drum-machine collection (Alesis HR-16B, Alesis SR-16, and others), a few pieces of seriously vintage/antiquated analog gear (Synare electronic drums, for instance, and a Mattel Synsonics unit), a Yamaha sampler, a Boss sampler, and most of the cool little boxes that Korg has come out with in the past several years (minus the Electribe, just because I have enough drum-machines already)-- The Monotron, the Monotribe, the Kaossilator, the KP3 mini Kaoss Pad, and the Kaossilator 2. One item that might interest you that is part drum-machine, part synthesizer, sequencer, sampler, and fx box is the Korg Kaossilator Pro. It does a bunch of things pretty well, is pretty ergonomic to use, and is fun, plus it has flashing lights, which everybody loves. You could begin to practice making beats, use it with your DJ gear, and it is portable enough to take with you on trips or to work in the car (when someone else is driving!), or whatever. It would also be something that would integrate well with other gear later. Eventually, you are going to want to record all of the beats and live performances that you do with this stuff, so you'll have to decide whether tracking to a computer-based DAW or a stand-alone unit of some kind is for you (or, if you really, really want to, some kind of analog recording machine, which is still out there if you must have tape).
Another way you could go, if you already have an iPad or an iPod, is check out some of the apps now available that emulate some of the vintage and new analog gear; that could be a way to spend just a little money to "try before you buy" and then see which types of gear you'd like to try with a more full-size hardware version.
God Bless and I hope you find exactly the right "thing" to get you started on this fun avocation. One word of advice-- shop around, take your time, ask a _LOT_ of questions, read the boards here and as many music/production/gear magazines and websites as you can; do your research and get to know what it is you really want to do before spending money on something that may not be what you want/need just because someone at GC or Sam Ash or wherever says "You must have this thing..." think about what would work for you best, what you want to accomplish, and what would be the easiest way for you to start by transferring your already developed DJ skills to this new venture. THEN spend the money to get that thing.
GJ