I shoot a lot of art for a number of clients, galleries & artists. If the works can be described as having dark, glossy, heavy impasto surfaces, I quote at least twice as much as I would for completely flat work. It's a bitch.
For works up to a couple of meters on the long side, I use two Einsteins with the standard 8 inch reflector, with polarizing gels on the lights. The lights are set back 3-5 meters at 45 degrees to the work, just like on a copy-stand. I also have a polarizing filter on the lens. Shoot tethered so you can have a very clear look at what you're doing. Even a very small adjustment to the light alters the result. It's very important that both lights and modifiers are 100% identical. Make sure you can work in a BIG space.
If there are multiple works, set up an easel and bring the works to your setup. If you move your setup for each work it will take you weeks to do what you can do in a day.
If there are still blown highlights/catchlights in the dark impasto areas, they can be fixed in post in a variety of ways. On an adjustment layer you can hit "dust & scratches" and/or "despeckle" and brush in the trouble spots through a layer mask. Or you can use the color range tool to select the blown areas, feather a tiny bit and literally paint them in. Or a combination of all these things. Remember to just reduce the blown areas till it looks good, not kill them completely or you take away the fact that it is a heavy impasto work. It's a tightrope.
Color? Get yourself an X-Rite color Checker Passport and use as directed. Trust the numbers more than what you perceive. If the works are framed, watch out for shadows thrown on the work by a deep frame.
Dark, glossy, heavy impasto paintings are the hardest of all. Good luck. If the work is important, and you feel it is beyond you at your current skill level, it may be best to pass on the job, skill up and start pitching to prospective clients again later. It's an area that looks simple but is full of traps for the unwary and inexperienced.