rehab, construction, renovation

[Eli M.]

Dear BP community,

I've got a question here. When you are doing a full rehab (new floors, kitchen, bathrooms, windows, electric, insulation) in a single-family house, should you try to install new HVAC system, new furnace and a new water-heater, which is a very big expense, or if the old one is working keep the old one, and when it brakes install a new one later on? 

What is the most cost-efficient strategy?

thank you all in advance, 

E. K.