All depends on how the duct work is run. 

Due to the age of the system (16 years outdoor unit) (11years air handler) and the current EPA phase out of the old r22 refrigerant I recommend the heat pump system be replaced.”Neither company is willing to replace any parts on the unit due to the age and not being able to warranty them.Here is the scope of work for the price of $3750Install one new indoor air Handler with 10kw heat strips,Transition duct work to fit the new air handler,Install outdoor heat pump,Connect all high and low voltage wiring to indoor and outdoor units also to the thermostat,Install in-line PVC P-trapClear PVC drain line,Evacuate system and charge with refrigerant,Check/test system.Install Honewell digital programmable heat pump thermostatComplete system install will come with a 10 year all parts and labor warranty.What do you experts think?

Depending upon the roof truss's sometimes it's not real expensive to run duct work into the garage (but in this case it sounds like the existing hvac won't make that work). 

Some examples of improvements include: a new deck, storm windows/doors, wall-to-wall carpeting, duct work, and insulationHere is link to a great forum discussion regarding this topic of Cap Ex vs Expenses: https://www.biggerpockets.com/forums/51/topics/70013-techniques-to-report-capital-improvements-as-repairs

Just duct work.

Expensive to install all the duct work.

Home warranties often cover very specific items/components (HVAC, hot water heater, etc), not structural, built-in systems (wiring/plumbing/ductwork) or aesthetic items. 

When I went into the house after the A/C guys installed ductwork, there was exposed duct going through a wall about 3’ above the family room floor.

I'm renovating a 1970s 1600 sq foot rambler that needs all new duct work in crawl space and a 3 ton package unit.  

It is a total project, the only thing that has been done is plumbing, electric, some duct work for AC, and the foundation and frame of the house.