Episode #1.1 - S1-E1
Other mistake: Lord Grantham tells Violet that he wanted to put 'lights' in the bedrooms but Cora did not, but at the end of the episode just before he blows out his candle Cora turns and off camera turns off a light that "clicks."
Answer: Lord Sinderby purchased the house, furnished and appointed, from someone else, complete with staff that come with the home (some of whom aren't happy to be working for a Jewish family). He bought the title, as well. If the artwork featuring Jesus is valuable, he might keep it. Also interesting, his ancestors Anglicized their name, though he is proudly Jewish.
Michael Albert