I find that when Jane Austen has a character use the words 'sort of' of 'kind of' in qualifying a description of him or her, it often means that they either really don't care much about what the person is like, one way or the other, or they really don't know much about the person at all, but are trying to make it seem as though they do.

Examples:

From Emma

"Our little friend Harriet Smith, however, is just such another pretty kind of young person." (Mr. Woodhouse re: Harriet Smith)

"The friends from whom she had just parted, though very good sort of people, must be doing her harm." (Emma, re: the Martins)

"A young farmer, whether on horseback or on foot, is the very last sort of person to raise my curiosity." (Emma, re: Robert Martin)

The Coles had been settled some years in Highbury, and were very good sort of people.

From Mansfield Park:

"Mr Rushworth is a very good sort of young man, and it is a great match for her."

From Northanger Abbey:

It was built for a Christchurch man, a friend of mine, a very good sort of fellow; (John Thorpe, talking about some person from college)

He seems a good kind of old fellow enough, and has lived very well in his time, I dare say; he is not gouty for nothing. (John Thorpe re: Mr. Allen)

I have a great regard for him, I assure you: a gentleman-like, good sort of fellow as ever lived." (John Thorpe, talking about how well he knows General Tilney)

"Her mother is a very good sort of woman," was Catherine's answer. ( Catherine, answering a question about Mrs. Thorpe)

From P&P

"But every body is to judge for themselves, and the Lucases are very good sort of girls, I assure you." (Mrs. Bennet about the Lucas girls)

"Chuse properly, chuse a gentlewoman for _my_ sake; and for your _own_, let her be an active, useful sort of person..." (Lady Catherine's description of Mr. Collins' ideal wife)

In most of these cases, the speaker or thinker is in almost total ignorance of the person they are describing, or they don't care anything about the person they are describing and the qualifier 'sort of' or 'kind of' is a clue. This is not to say that their is any animosity towards the person being described, but perhaps their personal qualities have never been considered before, or the describer just simply doesn't have any idea and doesn't know what else to say.

So, when Sir John says that "As good a kind of fellow as ever lived, I assure you," it makes me think of all these others who are kind of/sort of described, but not.