Some basic distinctions between manufactured and modular... I think you will see why Daybell was planning to buy a manufactured home.
A manufactured home is considered a mobile home. It has to follow HUD's code/regulations for mobile homes. It gets financed like a mobile home. And many subdivisions prohibit them. There are few options (kinda like buying a car). They radiate "budget". The terms "double wide" and "single wide" pertain to manufactured homes. The homes you see tooling down the interstate are almost invariably manufactured homes.
A modular home is simply a building technique. You can get several-storied office buildings and horse barns that are modular. They can be luxe models or basic, for the wealthy or poor (in my state, there are million dollar homes and Habitat for Humanity homes built modular). They are built as boxes and pieced together on site. They can be as custom as any house. They are built off-site and trucked in. (This is a huge advantage in my state that has winters go to -20 below, since the boxes can be built indoors.) They are placed on foundations.
After modular homes are placed, they are often indistinguishable from homes that are "stick built" (the way most homes are built, where they are erected onsite). Modular homes follow standard codes and get taxed like regular homes.
I happen to have a modular home and worked on HUD's mobile home regs.
That's the long and the short of it. But you see how you can be certain Daybell was going to get a manufactured home and not modular? There's not a snowball's chance in heck he would have had the money for anything other than a manufactured home. Not only that, he wouldn't have had the imagination to wrap his head around "modular" or anything involving "design".
So, yes, by necessity, the Daybell plan was to lay a concrete pad (I doubt piers were the plan given the circumstances) and place a mobile/manufactured home there.
Hope that helps.