How are you planning on scanning a large image with your scanner? Are you going to be able to reach all the image without folding it?
When we hire Illustrators, we have to keep in mind how we are getting their art scanned. Since all of the really large format flatbed scanners are pretty much gone (anything over 11x17,) and artists sometimes work in a medium that we cannot feed through a large document scanner (not that great a scan quality anyway,) we contract with a photographer who uses a very high resolution stationary scanback camera. We used to use 4" color chrome film and drum scan that, but that is pretty much dead too. The largest art our photog can do in one shot is about 36" x 36". He can do larger, but uses a custom stitching algorithm to do it (the name escapes me at the moment, but it isn’t photoshop.)
B’s and Steve’s advice is spot on. For something this size at the viewing distance you noted, 100ppi is definitely adequate.
Please do not forget your image bleed! When large prints are created, it is not an exact science as far as size is concerned. For something this size, I’d recommend at least 5cm all around for trimming and finishing. Your mileage may vary with that however, depending on how you are finishing this piece. For instance, 5cm would be enough for a standard pole pocket banner where the back is not seen, but if you are wrapping a frame to look like an art canvas and the edges are visible, your bleed has to also include the return distance of your frame. If the return depth is 10cm, your bleed would be 10+5 all around.
Keep any critical imagery/text out of this bleed/return area - and allow about the same amount of space inside your finished dimensions for a Safety as well, so no important imagery/text gets cut off. At 1.8 meters you can expect your print to be out of square +/- 1.5cm depending on what media you are using and the method of printing. Allow for some image to be trimmed.
The larger an image is printed, the more bleed you will need. It’s always best to discuss with your print vendor what they need for finish bleed. Think finishing before beginning.