The relationship between scanning resolution and halftone screen line values is well understood, and the calculations suggest that scanning at high resolutions is almost never necessary. After resizing the image to match your final requirement, you will need to have enough information in the scan to assure that a good halftone dot can be created at every point in the image. If all of your halftone screens were at 0 degrees, one scanned sample would suffice for each halftone dot. In any of these calculations, only the true hardware resolution is significant. No more information can be created by interpolation, so scanning at the higher values is mostly a delusion.
Dynamic range is the biggest single element.
This means the range of original colors, from stark white to dead black, that the scanner can identify and properly write to disk. The greater the dynamic range, the more likely you will be able to maintain shadow and highlight detail. The greater the dynamic range, the more different colors or grey values in the file, and the more enhancement is possible without posterizing other elements of the file. The CCD arrays that are used in flatbed scanners have much more trouble recording detail in deep shadow than in the midrange or highlight areas. Originals with significant detail in deep shadow are candidates for the service bureau, where the photomultiplier tube in a drum scanner can dig out more information than any but the best flatbed.