well, Access Virus, NordLead and Novation are THE synths for Dance Music (and not to forget the Roland JP 8000 which scooter also use), but they all are quite expensive. And one of the legendary Access Virus can only be bought second-handed, i think...
Are U sure, U really want a hardware synth? There R many good VST-synths as well and together with a decent MIDI-controller-keyboard (a MIDI keyboard with knobs and faders in addition to the normal keys) U can also achieve goo results...And it may cost less than a Hardware synth.
There R 2 types of synths: the keyboard-versions and the rack-ones. the rack-ones only differ from the others that they don't have keys, so U need another externel keyboard to play them, but on the other hand they are cheaper...
When it comes to VST-synths, i would reccomend the Vanguard ReFX. That is THE Dance/Trance/Techno-synth!
it is your decision, which cathegory U choose, but I am totally fine with vsts...