Repair of poor duct systems is labor intensive and difficult to do well. I would have a third party do a duct system pressure test to evaluate the exact condition of the current ductwork. Putting money into new equipment without the ability do efficiently deliver is a perfect example of throwing good money after bad. you have to be cold blooded in evaluating this.In some cases, like in my current home, I removed 100% of the ductwork and went with a mini-split heat pump. It provides heat down to 5*. Some newer models are heating down to -15f. My other heating appliance is a Rinnai Energysaver. The advantage of this combo system is I only heat the parts of the house I am using and there are no distribution system losses.