Hi @Danny Daniel
I agree with @Bill Jacobsen you should incorporate a 10% management fee even if you manage the property yourself.
As far as purchasing the duplex and running the numbers it simply depends on your minimum deal standards. Most individuals deal standards are different and will vary in someway. Be sure to have a clear minimum deal standard in order to help you with your decision, it will aid in pulling the trigger when you find a deal.
The typical and safe vacancy rate for a well maintained property in a decent location that is priced correctly is about one month per unit per year 8.33%. The vacancy rate in your initial assessment doubles this figure, its at about 16.5%. Which is fine if you want to be extra extra safe.
My assessment is as follows, assuming you paid the rehab and down payment with your own cash.
$2,100 - Total rent
-$708.60 - Principle and interest. (132,000 @5% for 30 years)
$165,000 - Duplex cost
-$33,000 - Down payment
$132,000 - Loan amount
-$275.00 - Taxes and Insurance (Estimated: dependent upon tax assessment)
-$175.00 - 8.33% vacancy rate (one month per unit per year vacant)
-$210.00 - 10% Maintenance reserve
-$210.00 - 10% Management fee (even if managing yourself)
-$100.00 - Utilities ( look up all utilities, especially ones that can lien your property)
-$80.00 - Lawn care or any other service
Equals $341.40 per month of cash flow. ( depending if you finance the repairs or down payment)
$341.40 X 12-months (With management fee)
NOI: $4,096.80 (With management fee)
$551.40 X 12-months
NOI: $6,616.80 (Without management fee)
If you paid the down payment and all the repairs with your own cash you would pay $53,000.
ROI: 7.7% with management fee
ROI: 12.5% with out management fee.
I hope you find value in this assessment.