Cash flow after operating expenses

[Doyle Brown Jr.]

Close to closing my first rental property deal. No money down. Owner is going to owner finance me for 12months @ 3.5% and give me deed. Im goinging to cash out refi with a traditional loan in 6-8months to pay the note off with the owner but also use the cash glow to improve the property leading up to application for loan with community bank. Its a large 4 plex in a B+ A- community. Rents are $500 per unit. One unit is vacant. Purchase price will be $60k though thou tax value is $108k. Its needs windows now and roofing in 3-5yrs. How much cash flow should I want out of  this property on a monthly basis given this scenerio? Thx for your responses in advance!