Especially in the current economic climate, I would knock 20% off the price as a starting point and find a reason for it after that.Not nonsense, if I viewed a 5bed property that is what I would knock of the asking price....
What you personally would bring as a reason makes little difference.
A condemned boiler knocks at best 2 grand off the price, a working combi is a warm house; nobody will reduce the price by whatever because you don't like the boiler.
How much would you knock off if it turns out they have a keston/ideal/jaguar or any other piece of junk?