Nest 3rd Generation and Aprilaire 700M

Been doing some internet browsing, searching about the best setup to humidify a 10 year old, medium sized 3 floor semi attached home in NYC (approx 2200sq ft).

All things bring me to the Aprilaire 700 series. Since I have a Nest 3rd generation thermostat on the 1st floor of the house I'm wondering if it makes sense to get the 700M model Aprilaire and have the Nest control the humidity or mount the external temperature sensor and manually control the humidity.

I have forced hot air and a Goodman gas furnace (see pics).

I have zero experience with these types of installs but was hoping to tackle this install with my dad who's a Plumber/Steamfitter (he know zip about electrical stuff).

How difficult would it be? Are there any A to Z instructions and wiring diagrams available?

Where would I mount the unit?

Is it better to get the 700M and lync it with the Nest or get a regular model with the exterior temp sensor and manual humidity control.

Looking for guidance from the pros as I hear a PRO install would be way more than the actual cost of the unit. Thanks in advance.

 

also I dont know if this info is needed to answer any of my questions but here is the wiring on my nest

 

The choice is yours. I like the separate humidistat with the outdoor sensor for better control but either will work. If you use the nest to control the humidifier you will need a spare wire from the thermostat to the furnace for the humidifier control.

I don't see much clear space on the supply register for the unit or the bypass duct.