How to match air travel to freighter departure

My wife and I have booked passage on a Polish freighter from the Netherlands to the great lakes. We have been advised that there will likely be only two weeks lead time for the departure. Is is best (least costly) to buy refundable or low cost change tickets or wait until we know the sailing date and pay the last minute ticket cost? We would prefer not to spend more than two weeks in the Netherlands before sailing. :)