Well, eventually, you'll need a more versatile lens.
On full frame Nikon the D750 you need a 14-24mm zoom.
On full frame Canon, you need a 16-35mm zoom.
On DX or APS-C, you need a 10-20, or 10-22mm zoom.
On Micro Four-Thirds, you need a 7-14mm zoom.
Just a couple of notes: All these camera formats are fine for real estate photography, since the images go on the web, are tiny, and will never be printed larger than about 6x8 inches.
The universal aspect ratio for real estate work is 4:3, making Micro Four-Thirds an obvious choice if you wish to "crop in camera." All 3:2 images get cropped on the sides, so keep that in mind as you work with your full frame. If you dive into the business, and want to keep using your D750 AND crop perfectly in the camera, you might consider a viewfinder mask laser-scribed with a 4:3 rectangle. Scott Helberg's the guy to do it:
http://www.viewfindermasks.comHe made portrait masks for me. Even if the D750 finder screen is fixed, Scott knows how to remove it and scribe it. He's been doing this for two decades.
Most of the real estate photographers I've known use an on-camera mounted strobe, bounced off of white ceilings, where available. They open all blinds and curtains, turn on all lights, and mix in the strobe.
You can choose a Nikon high-end model flash, or its equivalent from Yongnuo. Personally, I'd rather have a couple of Yongnuos, so I have a spare. Get the most power output you can afford. You won't always need maximum output, but if you start photographing cavernous mansions...
Some folks are smart enough to cover the strobe with an 85B amber filter gel, when the room lighting is mostly incandescent. They will sometimes use a CC .30 Green gel over the flash to match cool white fluorescent lamps, if that is the predominant lighting. Then they'll do a custom white balance (JPEG) or a click white balance (raw) off of a neutral test target image captured in the same light, to bring the light in balance as best they can.
You can buy sheets of Rosco or Lee Filters from most major camera supply dealers. Their web sites list ALL their options:
http://www.rosco.com/filters/cinegel.cfm?CategoryID=2http://www.leefilters.com/index.phpSome folks will carry a kit of several photo grade daylight fluorescent CFL lamps, which blend really well with daylight and strobe. If the client is willing, you can change out their lightbulbs (temporarily) with your own CFLs to render the scene as faithfully as possible. You can also use these lamps in clamp-on fixtures or soft boxes for supplementary lighting.
Such lamps are sold by Alzo Digital and others:
http://www.alzodigital.com/online_store/light_bulbs_compact_fluorescent-daylight.htmOh, you can also use a tripod and experiment with HDR or just use long exposures.
Good luck!