I'm actually surprised a main dealer is actually selling quite an old vehicle with high mileage and patchy history.
As pointed out above there is no such thing as sold as seen ( in a private sale maybe) so if everything else is fine I would go for it. If the vehicle develops a major fault within 6 months then you do have comeback and in my opinion a main dealer would be more forthcoming than an independant dealer.
Good uck whatever you decide