I remember during my years in the ad agency world that we paid quite a bit for drum scans. 'Course, the smallest we'd ever have scanned was medium format--and that was mainly for ROP (newspaper) ads. All of the food and beverage photography we shot for clients like Pizza Hut and Pepsi was done on 8x10 transparencies.
i don't know if I would recommended printing that large from 35mm. Even if you use a low ISO film. It would be considerably grainy. ISO is the film speed and the lower the ISO the less sensitive to light the film is which generally mean longer exposure time, which for taking some pics is unrealistic Like for action shots for where you want to stop action.
If you are serious of printing large I would consider switching to medium format. Even with using the same ISO film and printing on the same size paper it will looks extremely sharper.
I've made mural prints (30in x 40in) from a 35mm Tri-x negative. The images were grainy, but I like the textural quality of film grain, and push my film to enhance the grain structure. As long as the image is sharp, I believe you can use 35mm to get the large prints you are looking for.