Making money is easy for Finn, he just has to raid some dungeons.
KKW is a real estate mogul, so once Finn has the money he can procure the coastal lands the port would be built on.
Finn could probably convince PB to invest in it too and construction would either be contracted to the Candy Kingdom or done by the humans. Once basic living requirements are met the ship would be able to begin a shipping business to places outside of Ooo expanding global trade.
One of the things that also bugged me is the Business Men never got to come back, they would've been great as KKW's cronies. Imagine Finn hunting them down, thawing them out again, then giving them to KKW as a gift or something.
But odds are the season 11 comics will go in some other route and this doesn't feel all that in-character for Finn, so it's not really an idea I'd take to it's full conclusion.