You could try contacting some Quantity Surveyors or Architectural practices. They will have loads of drawings, but again due to client confidientialty may not wish to disclose drawings. Do you have to have architetural, structural and M&E?
An office building sounds like a good example to due, nice shape standard construction etc. You could use a text book to go thorugh the rough planning process, discuss things like location, ease of access in terms of construction, choice of construction, i.e. steel versus concrete, steel frame being faster than concrete. WIth regards to costings an initial cost estimate would be done first which would be reconciled on a cost per square metre or foot basis. Then as the design progresses costs are updated and a budget can be set against which cost plans are prepared.
The client brief could be quite limited as some clients do not know what they want. But you could describe that particular building as fitting the clients needs - i.e location, need within 18 month programme. You could also go into life cycle costs etc?
I could go on rambling but it does sound like one of those typical assignments, but your biggest challenge does sound like getting hold of drawings. Perhaps try some QS firms, small practices, they maybe willing to help.
Good luck.