interested or interested in

[Tukanja]

I came across this sentence

After the owner had shown him around the apartment, Shane decided it was just what he wanted, so he said, "OK, I'm interested. Let's talk turkey."

Is it possible to say the sentence this way below

After the owner had shown him around the apartment, Shane decided that it was just what he wanted, so he said, "OK, I'm interested in. Let's talk turkey."

[Josef Essberger]

"in" is a preposition

Prepositions come in front of* a noun, pronoun, noun phrase etc.
Prepositions
Prepositions Rule

That means the "in" must be followed by something like it, the apartment, your offer etc:

After the owner had shown him around the apartment, Shane decided that it was just what he wanted, so he said, "OK, I'm interested in it . Let's talk turkey."

* "I come from London", but in some cases the preposition may not be directly in front of its object, eg: "Where do you come from?" [very strictly and very formally "From where do you come?]

[Tukanja]

It's amazing how a well-made documentary can interest people in a subject that they've never been interested in before.

that or which?

I think both are possible. Depends on what speaker wants to emphasize.

but if the that is used what the it is substituted with?

[Josef Essberger]

Both possible:

It's amazing how a well-made documentary can interest people in a subject that they've never been interested in before.

It's amazing how a well-made documentary can interest people in a subject in which they've never been interested before. (slightly more formal)