You always underestimate yourself when you are one of the best photographers I've seen. They picked you because of your great portfolio. Relax.
Just call the client and tell them you want to make sure you provide them with the files they need. Ask to talk to whomever is doing post-processing and find out how they envision accomplishing this. (They'll probably be so amazed that a photographer actually talked to them before the shoot that they'll love you forever.)
You can explain that you see several possible approaches and you want to talk about how he or she envisions the final product. Personally, I'm not comfortable with the ND filter approach because it's kind of a one shot, roll the dice sort of thing and the risk of getting "ghosts" is really strong. (See the first ever recorded picture of a human being: Niepce's Paris Street Scene, where the guy getting his shoes shined shows up because he was standing still while the rest of the people on the street were moving)
I've never tried the approach recommended by Martin Evening, but it seems to me that would be a lot safer provided the person doing the processing understands what they are doing.
Honestly, I'm a little worried about not knowing their expectations. It's very possible that the only post-processing they intend to do is converting and tweaking the images for CMYK. He/she may have a heart attack if you give them 50 or 60 files that need to be merged without talking to them first.
As far as cameras, any full frame will be more than adequate. Heck, you could probably shoot it on a 7D and be fine. Remember, the major limiting factor is going to be the output resolution, not the capture resolution.
very good advice here…very good.