This page does not inspect your furnace. It will not tell you the unit is safe or unsafe. It will not tell you to open gas equipment, bypass a safety, or do hazardous work. Age and visible rust do not, by themselves, show a heat-exchanger failure.
Enter what you were told and what you were quoted. The result is a list of questions for a qualified technician.
Should I replace my HVAC?
This site cannot answer that for a particular house. A sensible choice depends on a written diagnosis, measurements, safety findings from a qualified person, compatibility, efficiency, parts, remaining life, and your priorities. Use the questions above, and get the answers in writing.