Number one--the Helix 5 and Helix 7 series are not linkable to any other units or each other. If you want to share anything with the Helix 10 series with a Helix model, it has to be at least the Helix 9 series. The connection is Ethernet.
Whatever model you purchase, it comes with all parts to make it fully functional for that model for a standard installation.
You may want to customize your installation for your craft. You did not state what the craft is.
Humminbirds do the Chart Position curser different than other brands. GPS cannot determine direction unless some consecutive track points are created in a certain direction within a certain amount of time. When going like .5 mph or less that is when that occurs. The other brands just have the position curser wander around on the Chart view. Humminbird's position curser turns into a O to indicate lack of direction calculation. For positioning on offshore waypoints, the Heading Sensor (compass) will indicate the direction the bow is pointed on the Chart view and eliminate the O curser so you will know your position in relation to the waypoint--that antenna is the AS GPS HS.
Some use the included gimbal mount, but most use an adjustable mount like RAM, Balzout, Johnny Ray, or Cisco.
The Helix 10 has built-in Auto Chart Live contour mapping and will record data in the unit's memory up to eight hours.
You can copy that data to a Zero Line card to delete the map memory and start over.
You can record sonar logs to a blank memory card for processing with the computer software Auto Chart or Auto Chart Pro.
There are installations posted on the Bass Boat Central Humminbird forums Installation and Networking topic for reference.