Getting into HVAC has crossed my mind if ever I lose my job. Lot more work physically compared to a desk job, but decent money, and once you know the systems it's fairly routine. Furnaces are surprisingly not as complicated as they look when you first open them. A/C units a re a bit more involved when it comes to dealing with the actual refrigerant, but probably not that hard with right tools.
I can't help but laugh at these companies that want to rent hvac equipment out for like $100/mo. "but if it breaks it wont cost you anything!" That's a big if. The pressure switch in mine started to act up last year, I was using crocodile clips to "kick start" the furnace (switch was stuck open, so it would fail the startup check) while I get time to find one. I got a new one form a local hvac place for about $60 and replaced it in about 15 minutes. As a HVAC tech I could charge someone $300 + parts + tax for that call.