5 year boat owner here, I've gotten by with a basic gen2 HDS7 + a fortrex for awhile but it is time to upgrade. I have an idea of what I want, ultrex + humminbirds with 360, but I'm not sure where I should go with the amount of sub models. I have an 18 foot fiberglass with a 150 on the back, for those with a similar setup, what works for you? voltage + shaft length preference on the ultrex? Preference on graph models? I'm looking to have one at the console and one at the bow, they don't need to be the latest and greatest but I'd like all the side+down scan plus 360, I know the transducer for 360 sells separately for around a grand. I'm aware of what Garmin has goin on with the force and livescope but want to go ultrex + birds.

Any insights are appreciated, thanks!

I depends on which 360 you want.
The original 360 works with any Humminbird that has an Ethernet connection since it is an Ethernet accessory. Those can be as old as 8 years. I have a Humminbird 1198c SI Combo that was made November 23, 2010 that I used with the 360 and several other models after that including the current models.
The new MEGA 360 will only work with current Humminbird models that have SI+ or DI+ in their model names. Those are G3N Helix units and G2 SOLIX units. That model connects to the unit's transducer port and uses the Side Imaging circuits.

Next is display size, get the largest that fits your budget. BIGGER is better for 360 and Side Imaging.
This is the MEGA 360 displayed on Helix 12 and a Helix 10:



If the Fortrex you have works OK, you can get the same voltage and shaft length you now have in the Ultrex.
The Ultrex is available in three built-in transducer choices.
1. US2 that does 83/200 kHz 2D sonar ONLY and is compatible with all but the newest units that have HW CHIPR for 2D sonar. YOu can degrade even the ones with HW CHIRP to run the US2 if you want.
2. MDI is MEGA Down Imaging with HW CHIRP 2D sonar, a current Helix G3N DI+ or SOLIX G2 unit is compatible with that and the MEGA 360.
3. MDI is MEGA Side Imaging with HW CHIRP 2D sonar, a current Helix G3N SI+ or SOLIX G2 is compatible with that and the MEGA 360.
You can just get the US2 version and put the transducer that is included on the trolling motor and not use the US2.

Very much appreciate the clarification, a lot going on with all these graphs. I almost bought a helix 12 chirp SI Gen2 until I realized it wasn't compatible with mega 360, this helps!

Correct me if I'm wrong, but if I get a gen3 Helix with chirp SI then I have to pair it with an MSI ultrex? Or can I get the MSI Ultrex, and then simply pair it with the head unit of a gen3 Helix with no transducer?

The Ultrex with a MSI transducer does the same thing as the SI transducer that comes with a G3N Helix SI unit. Soo you can use either the included SI transducer or the Minnkota MSI transducer. You can purchase the Humminbird without a transducer (CHO=Control Head Only) and just use the built-in Minnkota MSI transducer.

You can also dumb down a SI unit to be a DI/2D unit and use the Minnkota MDI transducer.
You can dumb it down even more to a 2D-only unit for 83/200 kHz and use the Minnkota US2 transducer.