Freighter travel these days is nearly always on fast container ships with a scheduled list of port calls. The only excepions I know of are Aranui 3 & the ship that services Ascension Island .
I did my training on cargo-liners in the good old days & they carried 6-12 passengers in good spacious cabins. Port time in those
pre - containerisation days was quite long. Not like today where you may only get a few hours in the middle of the night.
There is no doctor & a medical cert. would be needed.
There is no entertainment except the occasional film show.
There is much more interaction with the officers & crew.
Visits to working parts of the ship would be available.
Food is usually good but not so much choice.
There are various web sites about freighter travel booking & experiences.