To answer your last question, modular construction is the future of construction. The benefits of off-site fabrication are plenty: There's no inclement weather to worry about, there's more efficient sourcing and utilization of materials, less material waste in the factory, less site disturbance, less neighborhood disturbance, less soil compaction, less erosion from construction activity, better quality control, larger purchasing power, there are many more.
The modular fabrication market in the United States is disturbingly inadequate compared to what's happening across Europe and Asia with the adoption of robotics and automation in their production. We're even behind in things like building codes that lend themselves better to off-site prefabrication. Things like concealed electrical connections, mechanical waste line ventilation and foundation anchoring mechanisms that are common in those areas simply aren't allowed by the IBC. None of this is any reason not to consider modular building for your project.
I've been working in designing modular buildings for the last 15 years. My parents who founded the firm I work at have been consulting on modular projects since the late 80's. A few years ago we started developing our own projects and built a small 3-unit project. We're currently developing an 18 unit apartment building that will be prefabricated. Here's the basic gist: Your project size at 40 units is probably enough to get a manufacturer interested. Manufacturers generally want a large number of units to produce because repetition and quantity lend themselves well to the manufacturing environment. With a larger amount of mods you may realize some economies of scale where you will see a cost savings. I generally advise folks not to expect much more than a 5% swing in either direction. Where modular construction really shines is in the speed of construction which can lead to less general conditions, less financing and quicker time to market.
I did a write up of my project here if you're interested in reading about it. Good luck.