Advice on how to get reliable estimate on a commercial rehab

Hello! New to BP and my first foray into the forums and looking for advice.

I am eyeing purchasing an old building and turn it into a medical office (It's pretty much just bones now). My prospective tenant has a hand-drawn sketch of what she would like the layout to be. Since I would need a good idea of what my upfit cost would be before I offer on the building, I was wondering how much do I need to do to get an reliable estimate. Can I just take the sketch to a contractor? Do I need an architect? Any advice would be greatly appreciated.

Thanks!