There is an unspoken but predictable truth with any project bankrolled by the public purse: it will go over budget. Exponentially. So whether the existing 24 Sussex is renovated/restored, or whether it is razed/rebuilt, expect the price tag to be significantly higher than whatever the government says it will cost. 
As evidence, I present the Liberals' $2million firearms program, which became a $2billion boondoggle, with spending so poorly managed that the Auditor General could not ascertain its true cost.