Hi there, Kimmy......
With the complexity of that banner image, it would be best that your client stick with the four color CMYK process.
I can surmise the reason for your clients needs - bright, vibrant colors. But spot color printing has a smaller color range. Hence, the hotel and marina scene will turn out inaccurate and dull. Spot color printing does not require full color images, such as for business cards, simple stationary or the 3-color Royal Pacific Logo.
With CMYK four color process, you can print all colors in the color spectrum which are needed for the full color imagery in the banner.
I would suggest that your printer bureau do test prints for you before final print so you can check for yourself whether their alignment of the CMYK plates are accurately done. Inaccuracy of plate alignments cause fuzziness or sometimes color discrepancies.
Depending on the capability of their presses..... on a 2-plate press, print a test output of the first and second color plates.... check output.... then a test with the remaining plates. Problem here is that you have to wait for the presses to complete the printing of the first run before you can do a check before the second run. It's much quicker if they have a 4-plate color press.
Of course, you have to be present during production......