So far, we are thinking of concentrating on Mandarin, Julington Creek, Beauclerc and San Marco areas.

Concentrate on education for now while saving money.

But at the end of the day many times I thought if I just concentrated on listings only and spent as much time and money and effort on those.

My point is, if you can squeeze it on your budget and still happy with the profit, then go concentrate on full time work AND scale the business.

Concentrate on local banks or credit union for residential loans if you pursue a duplex. 

Individuals usually are lower traffic units maybe duplex, 4 plex, single family homes.Were a property management company has usually lots of tenants in a very concentrated area.Individual don't have as many amenities usually like a pool or work out room but peace and quite are priceless.I think each can be as attentive as the other to their residents needs.

Lenders will look at the concentration of your properties in a commercial loan as well, but the reserve requirement tapers off because if you have experience they know the loss of all rents at the same time is unlikely.

Which field should I concentrate so I can make huge profit?

We are staying in the Atlantic Beach area, which I understand is where the Black Bike Fest is concentrated due to the black history of the area.

Those are ambitious plans but it sounds like you have some focus.If it were me, I would concentrate initially on paying down your own debt while you continue to learn about the real estate market.