the exhaust that is on the bike is the originl exhaust , its pretty crusty underneath and i definately think its going to need replacing sooner rather than later , a stainless delkevic full exhaust system would cost around £700 !! So unless the present owner reflects this in a final price i may have to pass on it ,he's wanting £2500 for it at the moment
The part that will rust out is the collector box between the exhaust pipes and the silencers. If you have permission you could tap it with a wrench to see how firm the metal is, or isn't.
That's a higher price than we would see for any year of ST1100 over here, but UK prices on everything are higher anyway, so maybe £500 less would be fair.