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What is the model numbers of the antennas that you would like to reuse?
Both the 33 and the 39 will do what you are looking for and are removable if mounted on a gimble bracket. The screen size is kind of on the smaller size at only 5" but it is a true nav instrument. If you were looking for one that would do depth, fish, and nav, there are several more options and most now have installed GPS antennas so there is no need to an external one.
The screen size of the GP Is was going from memory and for some reason was thinking it was a 5" screen.
Your 1870 can be taken inside the house, powered up and then you can use the map and the curser to plot out a cruise or even mark dive spots.
The 33 and the 39 do still take external antennas. The one you have is a 12 channel, but should you choose to up grade, there are a few out there that can do 16 channels.
Yes the maps are upgradable. You would want the C-Map 4d+ map pack. You can get one for the are that you boat mostly in and it will show different depth changes, ridges and possible even tide information