It sounds like you are buying from a private person, which gives less consumer protection. bs' is right about the coolant level can be due to not being bled probably (start engine with expansion bottle cab off and run until cooling fan starts. When cold refill and keep an eye on cooling level afterwards). But still it could be a regular leak.
As the cam belt change is very important I would like to see the bill as proof and what have been changed besides belts and pulleys.
After 12½ years of ownership of a 156 2,0 TS all I can say is it was a joy and it was very reliable.