need help to know if this is a good deal

Hi, I'm very new into learning about real estate so forgive me for any silly questions, 

I came across a property in a small town anthony NM, it is a very small duplex with two apartments in the back (total 4 units) 

1st unit is decent, 2 bed 1 bath (decent condition)

2nd unit attached to first unit is basically a studio (very small)

3rd unit in the back is a 1 bed 1 bath studio

4th unit in back is 1 bed 1,bath unit. 

the owner lives in the 2 bedroom unit and his mother lives in the studio next to him. 

I've known him for years and he's rented out the back 2 apartments for 400 each with success, hardly ecer is there a vacancy (even tho they are pretty ugly and old) 

so long story short. I figure the 2 units in the back net 800/month

the 2bed unit could net 600

and the studio next to it another 400. 

total of 1800/month 4 units. 

they are old, they aren't the nicest thing and it is in a very poor area (tho sade, no crime, lots of older people, small town) 

he is asking 80,000

according to my calculations that's around close to 600 a month including mortgage, ins, and taxes. 

id hire a property manager which would be around 180

and put away cap ex of 100

mortgage 600

cap ex 100

manager 180

total 880

and can produce 1800 a month. 

it almost seems to good to be true, is it? am I missing something?