When do you want to make a judgment on

whether this is a good investment? For example, if you buy a stock at 3 and a year later it becomes 1 but two years later it becomes 5. If you look at it after one year, it's bad. If you look at it after two years, it's good.

You also need to know the expected appreciation/depreciation between now and the judgment time and the expected return/loss on the alternative use of the down payment.