New Mitel phones are gigabit compatible (5320e, 5330e, 5430e and 5360).
I would not go with separate cables for voice and data. Your phone and your PC can share the same cable, since you can connect your PC behind the phone. Both will work at 1 GIG.
Remember, if you want to separate voice and data, you'll have to double the number of cables.... and switch ports! This can become very expensive.
QoS is pretty reliable now and there's no reason you would separate voice and data in 2 different cables.
Anyway, since you're moving to a new building, you could buy 2 cat 6 cables per office. Second one only for future expension and not connected to a switch.
Hope this help!