Sale at 74A Conselyea Street

So does anyone have a good link or something walking people through the math to look at when considering a townhouse with two rental. Assuming you could get the 4300 per month for the two apts how much do you need to set aside for repairs, insurance etc.

the listing says "The costs of living at 74A Conselyea, with a 20% down payment, factoring in all carry costs and then the rental income, is an astounding $3,735/month." I doubt that is actually the case though. I have not been able to find a good reference to figure this out and I don't think the seller's agent is reliable in a case like this.