Hi Sam;
Sounds like it might be a good deal.
I will guess that the water jacket is made of
cast iron. If so, it can be welded but only with the correct rod and the correct person using that rod. Not just anyone can weld cast.
I would make some calls locally beforehand and see who can weld it for you and how much it might cost.
The add already says $1200 OBO so hes willing to dicker. If the welding costs is $100 est. then offer him $1000.
If you can't locate a welder locally. Then go see the boiler in person. (if its not to far)
It might be a mess with poor, old repair attempts. Or it might be a simple fix.
The owner might even know a place that could repair it and just doesn't want to deal with it.
Certainly worth investigating more!