What you may not understand is that the current system is set up for the EXACT configuration that currently exists in your home in respect to your ductwork and location of the furnace, and how this relates to the outdoor unit and the refrigerant lines that connect the outdoor unit to the furnace.

At best, you will need to make ducting changes, put in new refrigerant lines, and a gas line for the furnace.

You will probably not be able to put the furnace in the garage due to code. The garage is a potential source of carbon monoxide and shouldn't be tied into the home air supply.

At the most, you need a whole new system and a lot of new ductwork and new gas and electrical line and new refrigerant lines.

A lot depends on the age and configuration of the situation.

Could you more cost-effictively build a mechanical room space around the furnace that provides enough space for the safe operation of the equipment, and then sound-proof that to a satisfactory level? You may also need to address noise from the lines.