As soon as you up-grade the panel, everything after it would need to be brought up to code.
I am assuming that with knob & tube, the house has a 60amp service.
Before you buy the house, you might want to find out from the local Building Inspector, what the history of the house is. (permits for work that has been done)
150 yer old houses are great, tons of projects to do! What are the water lines made of, lead?
I would be surprized if you find a "Home Inspector" that can give you a though inspection of a house of this age.
I would not be too concerned about the horsehair plaster. Plaster, in and of it's self, provides a good barrier. I am surprized that the walls are not insulated with horse hair? That should have been about the right time for that in N. E.
If you are within 1/4 mile of the coast, you are probably spending a few dollars for this house, (depending on the area).
Do your homework on this one before you buy and don't leave it up to a home inspector.