His original intention... (Spoiler)
...was to rip everyone off right? We learn in the court scene that Horace had previously conned people in his old town to give him money for investment and then ran off with the lot. So I'm presuming Horace/Jack planned to impersonate Sommersby in order to come up with the tabacco scheme and then run off with everyone's valuables - that's presumably why he took so long when he went to Virginia; he was having a crisis of conscience. But in the end he'd started to fall in love with Laurel, so he decided to buy the tabbaco and make an honest go of it.
This makes sense, as initially it wasn't really clear to me exactly *why* Horace wanted to impersonate Sommersby.