Do I need a Sekonic C-700 for accurate color with a Nikon D800?

The TL;DR of it is that the Sekonic C-700 Spectromaster will be an expensive waste of money for your use case unless you decide to ditch the D800 (or any other digital camera capable of shooting raw) for transparency film. You will get much more useful and practical results much more quickly and much less expensively using a colour target and profiling software, such as the X-rite ColorChecker Passport or the Datacolor SpyderCHECKR.

A colour meter is primarily a lighting tool, not a capture tool. You use it in combination with colour-correction filters (gels) and whatever white balance tuning features your auxiliary lights may have to achieve consistent light colour in the scene. Or aesthetically-pleasing inconsistent light, such as when you want to include incandescent practicals (light fixtures that appear in the scene) in an otherwise daylight-balanced scene. (Real domestic incandescents will appear far too red; they need to be gelled up to a higher colour temperature, or your "daylight" needs to be gelled a little warmer. High-temperature tungsten lighting is only a little better in that regard.)

This task has gotten more difficult in recent years, what with LEDs and broad-spectrum fluorescents and so on, each having a slightly different spectrum and apparent colour temperature. A simple go/no-go check for gas discharge lights, putting CC30 magenta tube wraps over all of the fluorescent tubes to kill the excess green, then treating everything else as if it were a black-body radiator (that is, the colour temperature tells you everything you need to know in a practical sense) isn't enough anymore. You need to see the spectrum, both to make sure that you have the desired energy where you want it, and to make sure that you don't have wild energy spikes somewhere you don't want them. (You'd need to use high-Q bandblock filters to tame those.) The old industry standbys, like the Minolta Color Meter III/IIIF or the later Konica Minolta CS-200 (about the size of a typical VHS-C camcorder), won't give you enough information to work with discontinuous spectra. Knowing the mired difference between two lights is enough to let you filter continuous-spectrum lighting, but it isn't nearly good enough for discontinuous spectra.

The combination of features in the C-700 — colour temperature both in Kelvins and mireds, CC calculator (so you don't need to look up the mired difference on a separate chart to choose a filter/gel), spectral display, CRI reading (basically tells you whether or not you can get away with a particular light as an illuminator, or whether you need to overpower it, swap it out for a better practical, or kill it), and intensity reading — can pay for themselves quickly when you're doing "big lighting" on location. That's mostly in the realm of cinematography and architectural and industrial photography. It saves a lot of time and test shots running around filtering all of the lights on set, or swapping out the lights that can't be filtered.

These are probably not the kinds of problems you need to solve. Unless you're doing a body art feature for Architectural Digest, examining the mutual influences of interior decoration on exterior decoration, so to speak. If you were shooting transparency film, where you don't get to change the white balance of what is recorded, you might want all of that information up front so that you could filter your lights and lens to the film, but with digital (or negative film, which allows after-shot filtration) you're not going to need it for small-scale photography. (Unless you've blown your entire lighting budget on the meter and are forced to use whatever mixed sources of varying quality happen to be lying around at the time.)

What you want is a way to ensure that the picture you produce is a reasonably accurate representation of what you were shooting. At least as a starting point; you'll probably find that objective fidelity is working somewhat at cross purposes to artistic intent, but having the truth as a basis upon which to embellish a tale will result in a better story. For that, you simply need to ensure that the picture is right rather than that the light was right, and a correct white balance and camera profile for the lighting that was there at the time will fill the bill. Either the ColorChecker Passport or the SpyderCHECKR (with their respective accompanying software) will let you do that quickly, accurately and easily — and at a savings of well over $1000 over the C-700 in dollar terms, and a lot of time saved as well.