I do not know your budget, but I can assure you an entertainer or seated coach of that vintage was rode hard and put away wet.
You may think you are saving money by buying a bus like that cheap, but you are going to be waaaaaay ahead finding a similar vintage motorhome conversion for twice the money.
All older coaches are going to appear like a money pit because unless the owner was very careful with maintenance the list of potential issues you are going to deal with range from tires to air bags to brake chambers to leveling valves to radiator problems to possible engine and transmission problems. In the house you could face A/C issues, refrigerator problems, generator problems, etc.
You can mitigate a lot of the costs of getting a coach in reliable condition by doing a lot of your own work, and if you believe in luck you could postpone all work until something breaks, but the bottom line is at the age you mention there are going to be needed repairs and maintenance unless you find that rare well kept older coach and are willing to pay a reasonable price.
The transmissions in those older coaches were fairly robust and if the engine was maintained it too will be reliable, but any bus used in commercial service is going to be worn out and maintenance in its later years will be band aid type just to keep it going without spending money.