Attic Renovation remorse?

[Matt Tang]

So I have what I believe is a very competitive offer on a large two unit. The current owner took very good care of the property. My bid price was based upon the assumption that I'd be able to finish the attic with atleast 700 sft with a full bathroom for $50k and potentially another $15k if I wanted to add a kitchen. I thought the estimate was conservative considering that it doesn't have trusses and has a 15+ ft ceiling peak, three double window dormers and access via two separate stairways. To make a long story short, my mother had her friend (who is a contractor for high end homes) visit the property. He thinks that it's going to cost $100k+ without the kitchen

and I'll get 750sft out of the project. Anyone here have experience finishing an attic? Am I going to have buyers remorse? (I'm fairly confident that the seller will accept my offer)