The easiest rule on investment is A) How much cash out flow you "invested" vs B) How much cash inflow you earned. Than do the maths of whether or not it's worthwhile.

Thus i your case, you need to know how much down payment you're putting in, how much installment you're going to repay etc....

To get the exact it varies on how one wants to treat the down payment.
For me, I would take it into my year 1.
i.e.:

(Monthly Rental Received*12) - (Monthly Installment Paid + monthly misc charge)*12 - Cost for acquiring (down payment & legal fees)
That would be your "profit" generated.

If your initial cost is high, you may want to bring it to 2 years instead of 1st year like what id done on the above example.
Like doing business, one has to have a target on when to recoup back the capital. Same concept here.

In my first sentence, I used the word "varies" because some treat the down payment separately. Some has the idea that the initial cost spend there would only be calculated at the end when one wants to sell the property. So it's different for everybody. For me, I would rather take it as my year 1 cost for better management of cash inflow and outflow.