Don’t worry about K3’s internal voices and polyphony. It does not at all affect it’s performance as a MIDI controller.
Since Sheriff already made a suggestion, may I join him and also offer a suggestion of my own. You said that you will have quite a bit of time before actually being able to play with any workstation and make your decision. Meanwhile, do not put that time to waiste and familiarize yourself with some basics about MIDI and how it works. It will definitely help you when you will start getting into using workstations and software synths.

-ED-