Need help eliminating and relocating a system

First of all… I would like to thank all those who come to this website and lend their vast experience and help to this industry…especially to homeowners such as myself who are tired of getting different opinions on how to do a project. You guys have been great helping me with my 5 ton relocation to the other side of the house.

I am in phase 2 of my new garage project…..no I am not heating or cooling my garage..

This is what I currently have..,

A 3 ton first company pancake unit. In between floors…no access above.

This unit is very loud when it comes on it’s 20plus years old. Showing signs of age and R22 is more than gold

I would like to relocate a traditional air handler in my newly built garage ….hung horizontally to save floor space

I have attached pictures of what I think would work.

Please reference the pics

I have put tape lines inside my bathroom which is (inches away from the first company unit ) where I would soffit down the ceiling. …..pass the supply and return into this space …punch through the exterior wall into my new garage where my air handler will hang above.

I would remove the guts of the first company….retain the same return air location …keep the current distribution box/plenum in place and have the new supply duct feed into it which would then distribute the air into respective rooms. …(I hope I’m using the correct terms)

I have attached the exact measurements of my soffit area…and my wall penetration dimension into the garage where the supply and return ducts would enter and exit.

I also found a stock picture on the internet of a horizontally hung air handler for what I’m envisioning…..the only difference being the supply and return will be coming down from the A/H rather than up…

I am planing on replacing the old equipment with a 3 ton ….14 seer heat pump

I live in Phoenix..

Question….

Can I use 14inch supply and return…….it’s a total of 12 feet from where the A/H will be located to the distribution plenum located inside the house…

Or should I use 16….. …will be insulated flex either way…transitioned to metal elbows etc at both ends

If I go with a 16seer…do I need 16inch

Anything else I should consider?

I was hoping to find a ducted mini split but I don’t think the configuration and space constraints would work….

I would need a 36k ducted mini split…heat pump .and I can’t seem to find one…suggestions? With a reputable brand

The home has 2 systems. This one I’m currently working on is the downstairs unit and it really does not come as often because the house is open floor plan and the upstairs unit floods the house with cold air