HP Reverb G2 has a 2160x2160 full-matrix LCD. I would say it comes very close to being usable as a virtual monitor, but the virtual screen still needs to be quite large. Extrapolating from this and older generation headsets, I would guess 4K per eye should be enough for a true office experience.
On top of that, the headset itself must be comfortable to wear for sessions longer than 30 minutes. None of the current models are that comfortable.