Ecobee install with AC system disconnected
For now the only wires that are needed are Rh, W and C. The G terminal operates the fan. It sounds like you don't want to be operating the fan. You can simply connect the G terminal and use the fan at your discretion. Leave the Y terminal disconnected. If and when you want A/C you simply pull the stat off the wall connect Y and reinitialize the ecobee. When they start they check what wires are connected, and if you have a wire connected to it that doesn't go anywhere it may cause minor problems for the ecobee.

In regards to the A/C replacement referring to Jim Stewart's comment. A new air conditioner will be using 410a refrigerant. You don't want to reuse the old copper lines because


Lines used on r22 are not compatible with 410a
If the old unit was 410a, the lines need to be sealed from atmosphere when disconnected.
Flush kits are available to clean the lines (I would never use one, I always replace the lines)


So the cost of replacing it would be very close to the cost of a new installation. The electrical has been run. But it may not be done right.