You used the term historic, so I will mention you want to make sure its not on any registers or you will pay a lot more than $5000 after you tear it down.
Beyond that, the oil heat unit indicates a possible oil tank that will need to be abated and removed or filled with sand and sealed according to the rules and regulations of your city (contact the city for referrals for qualified abatement contractors for a bid.)
The "historic" also indicates the possibility of hazardous materials such as lead paint and asbestos. Certain fixtures like old pushbutton switches and double hung windows can bring some money at salvage to offset your costs.
Again, I don't know exactly what you are tearing down, how far away from the road it is, or what other materials besides wood are in the building. As far as a straight wood box like a shed, read the thread from about 4 days ago and get a chain saw or a bobcat and a sledgehammer. Your building sounds more complicated, and I would use some caution before I started demo.
On the face of it, no, $5,000 does not seem unreasonable. But I would get another estimate just to be sure, and make sure you know how they propose to dispose of the debris. In some cases you can be held responsible if they don't dispose of the debris legally.
Good luck, and post back with more info if you want a better answer...