I am in the same boat (well...kinda) I jumped in and started a business. Which in the short time has done fairly well, with a few hiccups. I however also have a degree in Advertising so that has helped with my business quite a bit too.
I have the same camera you do, except I have invested in some lenses (which help with image quality a bit.) Correct me if Im wrong here, but the D5200 is a crop sensor and ISO performance leaves a bit to be desired. When I get the funds I will be upgrading to a better camera (and all new lenses because its a crop) but you can do great work with this camera. Invest in some studio lights, backdrop frame, reflector board, and most importantly EDUCATION. That has been the one saving grace to my business, without it all my research on the internet does me no real good. I took the Professional Photography Course through the New York Institute of Photography and they have provided some essential tools (lighting) and business information. They step you through each type of photography and give you assignments so you have good direction. The course cost less then my camera so it is very doable.