Depending on where you live I guess is the big questions.
My house and hvac system is only 10 years old, it is a 10 seer system, I am installing a goodman 18seer system.
I was only going to replace the A/C part of it, but the newer models use R410 refrigerant and also have multiple stages that your old furnace can't control.
After some research last week, I am going to change the furnace and A/C unit along with getting a smart thermostat.
I can get the whole unit for under 4K and I plan to install it myself. I did get a quote of 2100 to install in, not a bad price in my opinion, but I will do it myself with the help of a technician.