The house is a 1500sqft double wide, built in 1992, and in fairly good shape.

Well, there are a vast array of hypotheticals:1.)

You would need to find out if it is an actual double wide manufactured home or a Modular home.  

It was a 15ft. wide x 40ft. deep rectangle and had value only because it was adjacent to my sister's home in Provo.

If the buyer will use it as an investment (rental) property, then it's wide open and you can do whatever you negotiate.

I know a good GC that offers a wide range of contracting services (from Interior Remodeling to Outside).

For example, if I want to find wholesalers to work with in Milwaukee, do I just cast a wide net on the forums here at BP?

Louis rent out the detached garages for extra income; your space would be a little more secure than a standard garage if you build it out with a wall and extra wide (or French style) double doors.

Holy moly ... so much to learn and re learn and never learn.but short version is its all regional.. you cannot ask this question on a nation wide forumwhat works in one jurisdiction does not work in another. your best bet is to talk with a local civil engineer that does lot developemnts.. they will be the go to person.

I am used to dealing with LVLs or gluelams at that span and they are usually either 4 @ 9.25" LVLs, 3 @ 11.5" or a 7" wide by 9.25" tall gluelam.