@Immanue Sibero
Cap Rate is only an information rate basically using the original sale price divide by the expected NOI, So is only a measure of Expected Return. (SALE PRICE/ YEAR NOI = CAP RATE)
So basically is a rule of thumb, as all rules of thumb, it will only tell you if you are going on the right direction.
I woudl recommend definitely to calculate your Cash on Cash Return and your Total ROI. Cash on Cash will let you know the rate you will get ON CASH for every dollar you put in to the project, and your Total ROI will include the Cash on Cash but as well will take in to consideration Appreciation, so will give you a better measurement of your return.
Now, to answer your last comment. Well the value of the property changes a lot if its financed or not. The price does not change for the SELLER, but it does for the buyer. The seller gets what his asking for but the buyer has the option to use Other People Money to pay for the property. And to start with, Other People Money has inters (the cost of this money), so definitely you are not pinga the same amount of money that the seller is getting.
This changes everything, you have a Mortgage on one side but you have as well Acurde Equity at the same time that you have to account for, so the published selling price doe not tell you much at this stage if we are speaking of returns. You are probably using a 20% down payment (so your personal investment is way lower than the published price) and if its a rental, then the tenant will be paying for your Mortgage, so, in summary, you are getting (if the project is a good one) some net chas each month, as well as some Acurde Equity and Appreciation, so the way to go to calculate if its a good business or not is by getting first a Positive Cash on Cash return and as well a Total ROI that meat your investment criteria.