When looking at office buildings, what property class do you usually target (A, B, C, D) and why?

I am currently working on defining my search criteria for an office building, and I am inclined to go with a mid to low-grade B property.  My reasoning is that I want a building with good curb appeal in a desirable area (not only will I be using it as an investment, but I plan on moving my CPA firm into that building).  However, forced appreciation is important to me (I want to be able to refinance within a few years to pull some money out), so I need a building that has some issues that I can fix to improve the value.