It's time for a new water tank & HVAC system for my 65 yr old house

[MerryHolidays]

2. Your HVAC might be fine. Why do you want to replace it?

The water heater is a different thing altogether, as 20 years is generally outside of the lifecycle. Rust and other stuff might happen to it and cause it to leak etc.

For water heaters, try something like this: https://www.thisoldhouse.com/plumbing/21015245/hot-water-heater-buying-guide

For HVAC, I would make sure you need one. Depending on where you live, if it gets really cold in the winter and it's working, I would leave it until the spring. Annual maintenance usually works well in keeping HVAC's going a long time.

You could also try a home warranty as a hedge against something major going wrong with the HVAC unit. Some of these home warranty companies give low-cost annual maintenance of the units, which protects you and them.

Plus you generally get other coverage with a home warranty (plumbing, electrical, appliances etc).