The 4.5” echomap I linked to above is self contained ( for gps). I put one of those on my ski boat last summer, it worked great. Nice little unit. But not sure if it’s appropriate for your installation, depends if you are looking to flush mount something, or screw/clip it onto the ‘dash pad’ - because there is no way to flush mount this little guy.
Can’t answer for sure on the larger units, but my understanding is that external gps antennas are no longer needed. But can be used if desired, for better/ faster signal, and might be needed, depending on where the receiver is mounted.