Congrats on the new tractor. I've heard some quotes on here from members having it done at the dealership. But I don't remember any of them.
When I bought my M9540 I made a deal to get the 3rd function kit, 3rd rear remote kit and 2spd PTO kit at wholegoods dealer cost. Then I installed them. The kits are complete down to needed zipties. Detailed instructions with pictures.
The 3rd function was very simple to install. Comes with a different joystick handle that has switches. The tractor has a wiring harness plug-in already in place to plug the kit wiring into for power.
If you are mechanically inclined with standard tools you can do the kit.
Price the kit.
Then price the dealer install fee.
Then you'll know whether it's worth it for you to do it yourself.