I vote for waiting. I am in a similar situation. My single unit A/C and furnace is also 20 years old. When it has problems next time, I'll get it replaced. My estimate last year was about $7k. I have never had furnace problems, but the A/C has caused problems 3 times in the last 10 years...
Having just joined today, this is my first post. I am nearing retirement and looking for something to keep me busy and happy. I have decided creating a workshop in my garage is what I need to do. Having watched a lot of YouTube videos, I decided I want to restore and repair old tools...