The original Motif or the Motif ES? There's a difference.
If you're new to this world and are considering a Motif as your first synth, then you're buying a whole lot of technology for $700, regardless of version. However, go slow--you might be getting in over your head.
I'll share, briefly, a little of what I know about Motif, and then make a recommendation on how I think you might best proceed.
The original Yamaha Motif appeared around 2001, and the Motif ES is from 3-4 years later. While both versions share the same audio architecture, something Yamaha calls AWM2, these are both are full-fledged workstations, which in this context means they are synths containing both a built-in sampler and sequencer.
The Motif, like the Roland Fantom, Korg Triton, and Kurz PC2, are Romplers, which is a class of synthesizer that reproduces ROM-based recorded samples of various instruments and waveforms, and make these little recordings sound like musical instruments through the use of various techniques to shape the sound, for instance, envelopes, LFOs, filters, and digital effects.
The newer Motif ES has been upgraded over the original, and so has double the polyphony of the original Motif, contains 1700 built-in patterns (arps) compared with 256 in the original, has more builtin effects, and most importantly, has the ES Roms, which have more of what many consider to be the cleanest sounding samples of any rompler available. The ES Roms are larger, because they contain more audio data, but I don't recall the delta size of the Roms. You can look this up on either of the two main Motif websites: Motifator.com or Yamahasynths.com.
The Motif 8 has 88 weighted keys, while the Motif 7 has 76 keys and the Motif 6 has 63 keys. Both smaller boards have a key action of the non-weighted, synthesizer type. These numbers are true whether we're talking original or ES Motif.
If this is your first workstation, you're gonna be impressed, no, change that to overwhelmed, by the wealth of options and features in your synth/workstation. But you should know that the ES is already a discontinued model as Yamaha has just this year released the Motif XS, a bigger, better, more, and further extension to the Motif family.
If I were you, I'd be cautious about buying an original Motif, which is now two steps from current, and also could be going on 7 years old. Synthesizers wear out.
Instead of an original Motif, why not pick up a used Motif ES 6, for under $1000, and perhaps very near your $700 price point? Yamaha also makes reduced models of the Motif, like the Mo6/7/8, or the MM6/7/8. See Yamahasynths.com for a product list.
You will not be dissapointed with the sound, but you will be confused be the myriad features, while at the same time trying to understand how to apply envelopes to the pitch, filter, and amplifier, or when to use a compressor, or how to setup an LFO filter sweep--in other words, learning basic synthesizer concepts.
Motifs are notoriously hard to operate and the Yamaha documentation is very terse. I'm active on forums like Motifator where noobs complain with regularity over the lack of handholding in the Motif documentation.
You just might be better off with a easier synthesizer to start. Do you need a sampler or a sequencer? If not, then you may not need a workstation right out of the gate.
Also, you might try another vendor's offering. Roland manuals have a reputation for being kinder to noobs, ie, they are written with more explanation and greater redundancy. You might look at a Roland as a first synth, though I haven't read the MM6 or Mo6 documentation--perhaps they are more oriented towards the beginner? I really don't know, but I have seen Yamaha docs that are excellent. The docs for the AN/DX200 tabletop synths are far more accessible than anything I've seen from Roland, for example.
There are certainly plenty of synthesizer models to choose from, and you'd be wise to start doing your research before you invest your hard earned money. The big 4 manufacturers are Korg, Kurzweil, Roland, and Yamaha, by the way, and you really won't go wrong choosing a product from these four. But you really need to know what you're doing and what you want.
For instance, if you want to play new age music with lush pads, you can't go out and pick up, for instance, a Korg MicroSynth. This little box is not made for lush pads--on the contrary, this is a DA synth that's made for dance and electronic music.
Here's what I recommend since you're a beginner: Start at Tweakheadz.com--Rich, the fellow who runs that site has amassed an encyclopaedic collection of tutorials documents and reviews that are related the subjects in which you seem to have an interest: synthesizers, home recording studios, midi, digital audio, various kinds of gear, music software, and many other things. Reading for a few days in his site should give you a comfortable background and enable you to ease into this very esoteric endeavor knowledgeably and with greater confidence. Knowledge is power and will help you cut through the hype, of which there is plenty.
My best advice is for you to first make yourself ready as a knowledgeable consumer. So, don't jump the gun at this or that deal that 'won't last long at this price'. Deals are like busses, and if you miss this one, another will come along soon enough.
Good luck, take your time, and don't forget to have fun--isn't that why we're here?