This Sekonic meter is extremely accurate:
http://www.bhphotovideo.com/bnh/controller/home?O=&sku=460371&gclid=CNmooPf0kL0CFcyhOgodejgAlA&Q=&is=REG&A=detailsAs stated earlier, 2 strobes (I like Elinchrom Digital RX-1200s) with normal and wide-angle reflectors work great. You can always dial the polarizer on the lens up or down or rotate one or both gels on the lights to capture metalic details or highlights to accentuate textured brush strokes.
As much as I love my Nikon D800, I would never even think of using it as a repro workhorse. I guess if you only shoot a few pieces of artwork a month it's workable. My shooting space for copy work is mostly black. You'd be amazed by how much more more micro contrast a camera picks up in a room without any reflective surfaces.
I agree that collages, assemblages, and other 2 1/2 D artwork requires supplemental soft light. That is when the soft box comes out and additional exposures are taken to be used as layers for masking out harsh shadows in post. Meaning, several exposures and lighting schemes may be required to get an accurate read on a difficult artwork. Back in the day, I charged $100 for a digital capture of a painting 24" X 36" or smaller. The color was typically close to 95% accurate. If a particular painting had complications, such as metal paint, 3D elements, or glass I upped the fee depending on how critical it was to create an accurate reproduction.
I started out with an 8 X 10 view camera back in the early 80s. E-6 and a C-41 processors were in-house. I remember having to create masks with an array of film stocks to hold back or burn in detail. Every element was pin-registered. It was a tedious and costly process. The sad part about it was that inkjet printers were not ready for prime time and the fate of hard work ultimately depended on the press, paper stock, pressman, etc.
Ink jet technology is infinitely better than offset lithography.
During my digital days, I made decent coin when I kept the Epson busy 20-40 hours a week and had a steady stream of copy work coming into the studio. I had no problem trusting my instincts and turning down projects. Today, on occasion, I will copy and print an odd watercolor here or there. I do this as a favor for friends--seasoned artists who are satisfied with pleasing color rather than dead-on accurate color. In other words, so long as the repro looks good to the eye, they are happy.