I'm guessing you already have a lens picked out? If it's an M42 lens, then any camera body would work. It all just depends on your budget. For most situations, an M42 lens is a poor choice for Nikon cameras due to the extended distance between their mount and sensor. But since you'll be focusing for macro, and not infinity, even that would work. So I'd just look at general camera specs within your budget, and consider the sensor size and how it relates to your lens. In other words, I wouldn't use a micro 4/3 camera if your lens is a 150mm. Nor would I use a full frame if your lens is a 24mm. They could me made to work, I'm sure, but they'd likely be a whole lot of hassle.
The one thing I would recommend is getting a proper copy lens, or enlarger lens. Not all macro lenses work well for scanning film. In order to get sharp corners, you need a macro lens that has a flat field of focus at shorter focal lengths. Not all macro lenses do this, and even fewer do it well. I went through several before finding one that worked well for me.