I think you have pretty well covered everything. Nevertheless I would get a quote from the factory first. I did not understand "spray insulation" as the modules should already be insulated. When I built my modular 90% was done in the factory and the rest done on site. I am satisfied with the factory's quality.....most of my complaints and recalls was for the site work! Imagine this being magnified 10 times if site built! GO MODULAR!!!!

Depends what comes as standard features. If you are in a cold climate get the best insulation package you can get. In a hot climate the best ceiling insulation that they order.
If it comes with plastic faucets, spring for metal.
If the fixures don't have shuts off before each one, orderit with them.
If a steel house door isn't standard, try to get it as an option.
If windows are single pane, go for insulated.
The best "bang for the buck" are the things that will save you money in the future or make the home easier to live in and not necessarily the things that look pretty and shiny.