The equipment may be identical in both cases. The room you have to compare is not. Timing shapes the total more than most owners expect, so the decision day deserves a place on the worksheet.
What a forced purchase takes away
When a system stops and you need heat or cooling right away, your choices shrink. You accept the first available slot, compare fewer options, and may take the model that is in stock. The paperwork gets read in a hurry. Nothing about this is wrong, but it limits your leverage on price.
What a planned purchase gives you
A planned replacement lets you set the schedule, collect written scope from more than one source, and check the size and type of the new unit against the house. It also lets you ask about work outside the box before the crew arrives. That is often where price differences sit.
The cost drivers to list
Write these down for any replacement quote: the size and type of the new unit, the condition of existing ducts or piping, the electrical work at the disconnect or panel, the pad, the drain, and any work that changes the fuel or the layout. Ask each seller to name each line. A total with no lines tells you very little.
Where planning starts
The planning point is where you begin to compare, not where you are forced to buy. Use the gas furnace page or the heat pump page to find the typical service life for your system, and start the comparison early. The published figures sit in the ranking on the appliance ranking page, which is the right place to read them before you talk to anyone.