Been there - Cosco Tower is around 5 mins walk to/from the Sheung Wan MTR station (Exit E1, then cross the road).
Like others suggested, you can consider living anywhere along the MTR Island line. However for HKD8.5K I don't think Wan Chai, Causeway Bay, Taikoo is a good idea - space may be small!
You best bet will be:
North Point (but the buildings there are old, some of them more than 30 yrs),
Tin Hau (also very old buildings, although again most with renovated interiors),
Quarry Bay (similar old buildings but a good mix of new/trendy and old local feel).
I think that timklip's pointer of Heng Fa Chuen is one of the best option for you (5 mins waiting time on platform, 25 mins on-train time, and another 5 mins walk to office?).
Hope it helps
