Sorry but I'm a bit confused by your question.
I think based on the subject description and the first line in your post that you want to buy the actual dongle? But later you say "have a new (previously unfitted) one on our boat with a dongle".
So do you have the dongle or need buy the dongle?
Also, how much will you have to pay to download all the charts? If hundreds and hundreds then you might be better buying
a new plotter preloaded with charts. You can get a Garmin
740 (last year's model) new for $700-$800 if you shop around and it comes preloaded with all US charts. Not sure about how much coverage on the Great Lakes
but I can check mine if you're interested.