All you need is a keyboard that works. You press a key and then it sends a midi event to the synth. Stop. That's the only caveat you face when buying a used keyboard. Make you sure that keys are on line with each other and that all of them work evenly. As a general rule don't buy used synth. The internal battery has a lasting time and, unlike vintage violins or guitars, synths do not get better with age.
You need at least a 49 keys keyboard or more unless you need it only to make samples or are getting really short of space. a 61 keys is good for those if money is an issue, but nobody would want to run out of keys when performing at a wedding party. Still there are 88 keyboard performing piano action with keys precisely weighted, expensive enough and unuseful for those that are guitarist differently muscolarized than a classical pianoist. Nevertheless even if at this stage you are cornered on a few central keys maybe someday you'll feel to get out from there and use some more keys, just for a try: 88s keys with synth action are unbeatable if you are in hurry with a drum&bass piece and prefer a localized synth. Same problem if you are an alive performer and you don't want to take with a pc too. The arpeggiator wich strins notes when you press keys into a cadence is also an interesting feature, but don't bother if you don't have one, you can do without it. What you won't live without even on a tiny keyboard are the Velocity (the more you press the more the sound is louder) and the aftertouch (called also pressure sensitivity) wich gives a subtle variation more audible on pad than on piano or drums. A few later reccomendations: don't buy keyboards that misses mod wheel and pitch wheel they are essential, less needed al the theory of knobs many times duplicating what computers do better. A few words about the type of synthesis. Now if you are strictly doing Dance, Trance, D 'N B, Techno, then yes, the analog modeling synth will do the things you want that a sample playback synth cannot touch. This type keyboard is not for covering all instruments' like brass, guitars, pianos, organs, etc.  These are the modern day equivalents of old analog synthesis, which makes sounds by shaping a raw waveform with a filter and envelope.  While these modeling synths are "retro" in that they emulate the old beats of the past, they also dramatically extend them.  
But if you already have a virtual analog synth of quality, or a lot of "analog" software synths,  you might want to get a bread and butter sample playback synth.  If you don't want a multiple module studio and only want one keyboard and a computer and nothing else, then yes, you ought to get one loaded with options of different kinds of synthesis. On my site  I'm keeping up to date interesting offers on ebay.
Tina- The TechZone