I have a Garmin 498c and have been very happy with it. It is however; a discontinued model.
I too was disappointed that a PC interface was not available, but I wanted it mostly to be able to power the device while I learned how to use it. The alternative for me was to purchase an AC power adapter and use the simulator function, which works nicely. The AC cord also has a USB connector for use in uploading device software updates.
I have not used the SD slot for anything thing yet so I don't have any useful comments; however, an SD reader shouldn't be that expensive, my laptop and my HP printer both have built-in readers.
As far as your dash layout, I won't be of much help there either, my boat is a 195DC. Here is a pic of the dash anyway.
http://www.triumphowners.net/photos/...php/photo/2272