I can understand the desire for a scaled down Queen Mary, Mayflower, Santa Maria and a number of other notable vessels. Many have been scaled down and for many reasons, but a freighter? Now it was a sad thing with the Edmund Fitzgerald, but frankly untold thousands of freighters have gone down with all hands and not had a PBS special and top ten song done about the event.
Scaling down a craft as large as the lakes freighters to a 40'er would be a difficult thing to do well, though could be done, but it would only be a mild facsimile of the full size sister. Once you scale the length down to the 40' target the hatches, doorways and other deck openings become to small for practical use, a 6" tall doorway (at scale) for example, would have to be resized to allow a person to walk through it. It may require the height of several decks just to accommodate the doorway. This would cut up the "scale" appearance of the other items aboard, but can be hidden some what.
I bill out at $300 an hour for new design work, but generally work from a percentage of the value of the craft, some may work for less, others much more. For a ball park guess at the costs involved for the design work, take your highest intended budget for this project, double that, then the design fee will be about 3 to 7% of that figure. This is a very rough measure, but does work reasonably well at calculating the design costs.