Not quite as many keys as you want, and I haven't tried it yet, but the
E-MU Longboard seems to have a lot in it's favour. Built in sounds
from the E-MU library too, and all for less than half your budget.
It might not
be what you want, but definitely worth auditioning just to make certain it isn't.
Andy
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When the going gets weird, the Weird turn Pro.