In Tuck Everlasting, why does the man in the yellow suit go to the Foster's house?

Twenty years prior to the events of Tuck Everlasting occurring, the man in the yellow suit was told about a mysterious family.  The family that he was told about supposedly never got any older.  

". . . my grandmother told me stories. They were wild, unbelievable stories, but I believed them. They involved a dear friend of my grandmother's who married into a very odd family. . . This friend of my grandmother's, she lived with her husband for twenty years, and strange to say, he never got any older. She did, but he didn't. And neither did his mother or his father or his brother."

In addition to the strange details about a family that didn't age, the stranger's grandmother included a detail about a music box that the family had.  His grandmother clearly remember the tune that the music box played, and she taught the tune to the man in the yellow suit. 

"My mother was able to remember the melody, finally. She taught it to me."

That was twenty years earlier.  By the stranger's own admission, he tried to forget all about the stories of the strange immortal family.  He tried in vain though, because the man in the yellow suit said the story and the music tune haunted his dreams.  After twenty years, he decided to try and back trace the route that the family likely would have taken.

So a few months ago I left my home and I started out to look for them, following the route they were said to have taken when they left their farm. No one I asked along the way knew anything. No one had heard of them, no one recognized their name.

The above quote makes is clear that the man in the yellow suit is stumbling along an unknown trail in order to find clues.  He asks everybody along the route about the family.  The Foster family happens to be along the route, so he approaches the house in order to seek out more clues about the immortal family and the music box.  The Fosters seem like a good family to pick, because they have lived in Treegap many years.   

The man lifted his eyebrows. "Oh," he said, "I'm looking for someone. A family. . . This young lady tells me you've lived here for a long time, so I thought you would probably know everyone who comes and goes."

That is the first time that the man comes to the Foster home.  The next time that he comes is because he wants to trade his knowledge of where Winnie is for the ownership of the woods.